Example of a custom PMPro no access message body with a button link.

example-no-access-message-with-button.php

<?php
/**
 * Customize the no access message with a button.
 *
 * 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_no_access_message_body_with_button( $body, $level_ids ) {
	global $pmpro_pages;

	$body = '<p><strong>You must have one of the following membership levels to access this content:</strong></p><p>!!levels!!</p><p><a href="' . pmpro_url( 'levels' ) . '" class="pmpro_btn">Join Now</a></p>';
	return $body;
}
add_filter( 'pmpro_no_access_message_body', 'my_pmpro_no_access_message_body_with_button', 10, 2 );