Finding the screen id of a page generated with add_menu_page

I'm at a basic level of WordPress + PHP and I'm trying to build a pretty simple plugin.

I've set up the plugin correctly, and the plugin page is generated with

function wpplugin_settings_page() {
    add_menu_page(
        'Modal Generator',
        'Modal Generator',
        'manage_options',
        'modal-slug',
        'wpplugin_settings_page_markup',
        'dashicons-format-gallery',
        100
    );
}

Whereas wpplugin_settings_page_markup is a function that generates my HTML on the plugin page.

I copy-pasted a code to create/generate metaboxes and they work great. I can get them to show on posts pages with this code:

function single_repeater_meta_boxes() {
    add_meta_box( 'single-repeater-data', 'Single Repeater', 'single_repeatable_meta_box_callback', 'page', 'normal', 'default');
}

I achieved this by changing the $screen-parameter, as WordPress' official documentation states:

$screen

(string|array|WP_Screen) (Optional) The screen or screens on which to show the box (such as a post type, 'link', or 'comment'). Accepts a single screen ID, WP_Screen object, or array of screen IDs. Default is the current screen. If you have used add_menu_page() or add_submenu_page() to create a new screen (and hence screen_id), make sure your menu slug conforms to the limits of sanitize_key() otherwise the 'screen' menu may not correctly render on your page.

However, I can't get them to show on my dynamically generated plugin page. I somehow need to find the screen ID and use that as the parameter value for $screen. The slug modal-generator shouldn't be an issue. I hope my question makes sense and if not, feel free to ask.

add_action('admin_init', 'single_repeater_meta_boxes', 2);

function single_repeater_meta_boxes() {
    add_meta_box( 'single-repeater-data', 'Single Repeater', 'single_repeatable_meta_box_callback', 'toplevel_page_wp-nordic-modal', 'normal', 'default');
}

/* =========== GENERATING METABOXES =========== */
function single_repeatable_meta_box_callback($post) {

    $single_repeater_group = get_post_meta($post-ID, 'single_repeater_group', true);
    $banner_img = get_post_meta($post-ID,'post_banner_img',true);
    wp_nonce_field( 'repeaterBox', 'formType' );
    ?
    script type=text/javascript
        jQuery(document).ready(function( $ ){
            $( '#add-row' ).on('click', function() {
                var row = $( '.empty-row.custom-repeater-text' ).clone(true);
                row.removeClass( 'empty-row custom-repeater-text' ).css('display','table-row');
                row.insertBefore( '#repeatable-fieldset-one tbodytr:last' );
                return false;
            });

            $( '.remove-row' ).on('click', function() {
                $(this).parents('tr').remove();
                return false;
            });
        });

    /script

    table id=repeatable-fieldset-one width=100%
        tbody
            ?php
            if ( $single_repeater_group ) :
                foreach ( $single_repeater_group as $field ) {
                    ?
                    tr
                        tdinput type=text  style=width:98%; name=title[] value=?php if($field['title'] != '') echo esc_attr( $field['title'] ); ? placeholder=Heading //td
                        tdinput type=text  style=width:98%; name=tdesc[] value=?php if ($field['tdesc'] != '') echo esc_attr( $field['tdesc'] ); ? placeholder=Description //td
                        tda class=button remove-row href=#1Remove/a/td
                    /tr
                    ?php
                }
            else :
                ?
                tr
                    tdinput type=text   style=width:98%; name=title[] placeholder=Heading//td
                    tdinput type=text  style=width:98%; name=tdesc[] value= placeholder=Description //td
                    tda class=button  cmb-remove-row-button button-disabled href=#Remove/a/td
                /tr
            ?php endif; ?
            tr class=empty-row custom-repeater-text style=display: none
                tdinput type=text style=width:98%; name=title[] placeholder=Heading//td
                tdinput type=text style=width:98%; name=tdesc[] value= placeholder=Description//td
                tda class=button remove-row href=#Remove/a/td
            /tr

        /tbody
    /table
    pa id=add-row class=button href=#Add another/a/p
    ?php
}

/* =========== SAVING METABOXES ===========*/
add_action('save_post', 'single_repeatable_meta_box_save');

function single_repeatable_meta_box_save($post_id) {

    if (!isset($_POST['formType'])  !wp_verify_nonce($_POST['formType'], 'repeaterBox'))
        return;

    if (defined('DOING_AUTOSAVE')  DOING_AUTOSAVE)
        return;

    if (!current_user_can('edit_post', $post_id))
        return;

    $old = get_post_meta($post_id, 'single_repeater_group', true);

    $new = array();
    $titles = $_POST['title'];
    $tdescs = $_POST['tdesc'];
    $count = count( $titles );
    for ( $i = 0; $i  $count; $i++ ) {
        if ( $titles[$i] != '' ) {
            $new[$i]['title'] = stripslashes( strip_tags( $titles[$i] ) );
            $new[$i]['tdesc'] = stripslashes( $tdescs[$i] );
        }
    }

    if ( !empty( $new )  $new != $old ){
        update_post_meta( $post_id, 'single_repeater_group', $new );
    } elseif ( empty($new)  $old ) {
        delete_post_meta( $post_id, 'single_repeater_group', $old );
    }
    $repeater_status= $_REQUEST['repeater_status'];
    update_post_meta( $post_id, 'repeater_status', $repeater_status );
}

/* =========== PLUGIN PAGE ===========*/
function wpplugin_settings_page()
{
    add_menu_page(
        'Modal Generator',                  // page title
        'Modal Generator',                  // menu title
        'manage_options',                   // capability
        'wp-nordic-modal',                  // menu_slug
        'wpplugin_settings_page_markup',    // callable function
        'dashicons-format-gallery',         // icon
        100
    );
    add_submenu_page(
      'wp-nordic-modal',
      __( 'Popup Generator', 'wp-nordic-modal' ),
      __( 'Popup', 'wp-nordic-modal' ),
      'manage_options',
      'wpplugin-feature-1',
      'wpplugin_settings_subpage_markup'
    );

}
add_action( 'admin_menu', 'wpplugin_settings_page' );


function wpplugin_settings_page_markup()
{
    // Double check user capabilities
    if ( !current_user_can('manage_options') ) {
      return;
    }?
    div class=wrap
      h1?php esc_html_e( get_admin_page_title() ); ?/h1
      p?php esc_html_e( 'Some content.', 'wpplugin' ); ?/p
    /div
    divsave/div
    ?php
}

function wpplugin_settings_subpage_markup()
{
  // Double check user capabilities
  if ( !current_user_can('manage_options') ) {
      return;
  }
  ?
  div class=wrap
    h1?php esc_html_e( get_admin_page_title() ); ?/h1
    p?php esc_html_e( 'Some content.', 'wpplugin' ); ?/p
  /div
  ?php
}