How do I set a hidden field to a value?

have a WordPress site. There is a form on the product card, when filled in, the data is sent to the mail. It has hidden fields, how can I add the value for these hidden fields, which is stored in my variable? For example the key of my field: hidden_price And there is such a code, but I do not understand how to do it.

add_filter( 'ninja_forms_render_default_value', 'my_change_nf_default_value', 10, 3 );
function my_change_nf_default_value( $default_value, $field_type, $field_settings ) {
  if( 'textbox' == $field_type ){
    $default_value = 'foo';
  }
  return $default_value;
}