Adjust the Paid Memberships Pro login "Join Now" Link.

pmpro-adjust-the-join-now-login.php

<?php
/**
 * Adjust the 'Join Now' link on the login page to a custom URL for Paid Memberships Pro.
 * Follow this guide to add custom code to your membership site - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_custom_join_now_link( $links, $allowed_html ) {
	$links['register'] = '<a href="' . add_query_arg( 'level', '13', pmpro_url( 'checkout' ) ) . '">' . __( 'Join Now', 'paid-memberships-pro' ) . '</a>';
	return $links;
}
add_filter( 'pmpro_login_forms_handler_nav', 'my_pmpro_custom_join_now_link', 10, 2 );