Redirect away from the checkout page if a specific parameter is not in the URL

my_pmpro_require_url_param_for_checkout.php

<?php

// Copy from below here...

/*
 * Redirect away from the checkout page if a specific parameter is not in the URL
 */
function my_pmpro_require_url_param_for_checkout()
{
	$request_key = 'checkout_password';
	$request_value = 'my_password';
	// With defaults set above, a sample checkout URL that would be allowed at checkout is
	// www.yoursite.com/membership-account/membership-checkout/?level=1&checkout_password=my_password
	
	global $pmpro_pages;
	
	if( empty( $pmpro_pages ) )
		return;
		
	if ( is_page( $pmpro_pages['checkout'] ) && ( empty( $_REQUEST[ $request_key ] ) || $_REQUEST[ $request_key ] !== $request_value ) ) {
		wp_redirect( site_url( 'page-explaining-you-need-level-1-first' ) );		//change url here
		exit;
	}
}
add_action('template_redirect', 'my_pmpro_require_url_param_for_checkout');