Using a WordPress REST API route to proxy request to thrid party API, so api key isn't needed in client https://twitter.com/UpTheIrons1978/status/1602938132244094976

client.js

$.post( url_of_wp_endpoint, {email: 'roy@roysivan.com' } );

proxy-route.php

<?php

/**
* Endpoint for adding contacts to CRM
*/
add_action( 'rest_api_init', function () {
  register_rest_route( 'whatever/v1', '/add-to-crm', array(
    'methods' => 'POST',
    'callback' => function($request){
      $resoponse = wp_remote_post( $url_for_crm, [
          'body' => [
            //Pass auth token defined in a constant in wp-config
            'token' => AUTH_TOKEN,
            'email' => $request['email']
          ]
      ]);
      //Return a response...
    },
  ) );
} );