Redirect away from checkout if the secret URL parameter is missing.

my_pmpro_template_url_parameter_at_checkout.php

<?php
/**
 * Redirect away from checkout if the secret URL parameter is missing.
 */
function my_pmpro_template_url_parameter_at_checkout() {
	global $pmpro_pages;

	// Return early if we are not on a PMPro page.
	if ( empty( $pmpro_pages ) ) {
		return;
	}

	// Return early if this is not the checkout page.
	if ( ! is_page( $pmpro_pages['checkout'] ) ) {
		return;
	}

	// Get the level.
	$pmpro_level = pmpro_getLevelAtCheckout();
	
	// Return early if this is not level ID 2.
	if ( empty( $pmpro_level->id ) || $pmpro_level->id !== '2' ) {
		return;
	}

	// Get the URL parameter.
	$my_url_parameter = sanitize_text_field( $_GET['my_url_parameter'] );

	// Redirect away from checkout if the URL parameter is missing.
	if ( empty( $my_url_parameter ) ) {
		wp_redirect( pmpro_url( 'levels' ) );
		exit;
	}
}
add_action( 'template_redirect', 'my_pmpro_template_url_parameter_at_checkout' );