Default checkout level if no level parameter

default-checkout-level.php

<?php
/**
 * Set default checkout level if none is specified.
 * Add this code to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_set_default_level( $level ) {
	return 1; // Change the level ID
}
add_filter( 'pmpro_default_level', 'my_pmpro_set_default_level' );