Create new class object inside WordPress template

I’ve a problem. My plan is it to create a new class object inside a WordPress template to pass a variable from the post object to it:

<?php
/**
 * Template Name: Ticket
 */

defined( 'ABSPATH' ) || exit;

new Test( $post->ID );

Inside my class I’m defining an action that adds a AJAX function inside the class:

class Test {
	public function __construct( $ticket_id ) {
		$this->ticket_id = $ticket_id;

		$this->register();
	}

	/**
	 * Register all hooks
	 */
	public function register(): void {
		add_action( 'wp_ajax_test', array( $this, 'test' ) );
	}

	public function test(): void {
		error_log( $this->ticket_id );
	}
}

The problem is that the AJAX function is not reachable . Maybe it’s added too late? Because when I create a new class object directly in my functions.php file, it’s working . In this case I don’t know how to get my post id inside my class.

Thanks for helping me out!