wp_rest_registration_and_handling.php

<?php
add_action( 'rest_api_init', function () {
	register_rest_route( 'heartweb/v1', '/webinarData', [
		'methods' => 'POST',
		'callback' => 'heartweb_webinarData',
	] );
} );

function heartweb_webinarData(WP_REST_Request $request){
  $payload = $request->get_params();
  
  /*
    Now $payload contains array with request data from $request - no matter what type of request is handled
    Read more:
    https://wp-kama.ru/function/register_rest_route
    https://wp-kama.ru/function/wp_send_json
  */ 
  
}