Redirect certain levels to a custom confirmation page after checkout.

my-pmpro-confirmation-url.php

<?php
/**
 * This recipe will allow you to send predefined Membership Levels to a unique confirmation page.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_confirmation_url( $url, $user_id, $level ) {
	$level_id        = intval( $level->id );
	$redirect_levels = array( 1, 3 ); // level ID's of membership levels to redirect to custom confirmation page.
	if ( in_array( $level_id, $redirect_levels, false ) ) {
		$url = home_url( '/my-page-slug/' ); //change this to the url you want to redirect to
	}

	return $url;
}
add_filter( 'pmpro_confirmation_url', 'my_pmpro_confirmation_url', 10, 3 );