Post body not working with wp_remote_post()

I'm trying to post with with the following code

    $userTokenApi = 'https://api.mindbodyonline.com/public/v6/usertoken/issue';

    $args = array(
        'headers' = array(
            'Content-Type' = 'application/json',
            'SiteId' = '6387',
            'Api-Key' = '7bba39594b4d460293abdfd64c8eea48'
        ),
        'body' = array(
            'Username' = 'myusername',
            'Password' = 'mypassword'
        )
    );

    $request = wp_remote_post($userTokenApi, $args);
    $responseCode = wp_remote_retrieve_response_code( $request );
    $body = wp_remote_retrieve_body($request);

    if ( is_wp_error( $request ) ) {
        return false; // Bail Early
    }

    $pretty = json_decode( $body ); ?

But the response I'm getting back from the API is

Error:
Code: MissingRequiredFields
Message: The following parameters are required: Username, Password

The standard HTTP request for the same action (with PHP) is seen here https://developers.mindbodyonline.com/PublicDocumentation/V6#user-tokens and using postman I'm able to post and receive my response fine with PHP - HTTP Request2, PHP - cURL, and any other type of code.

I'm not sure what I'm missing here or what I don't understand in the documentation

Any help would be amazing. Live issue can be seen here - error's in console.