Load content on confirmation or checkout page for PMPro.

load-my-script-pmpro.php

<?php
/**
 * Load content for specific pages, checkout or confirmation page
 * Add this code to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function load_my_script_for_pmpro() {
	
	global $pmpro_pages;

	if ( is_page( $pmpro_pages['checkout'] ) || is_page( $pmpro_pages['confirmation'] ) ) {
	?>
		<!-- script or styles go here -->
	<?php
	}
}
add_action( 'wp_head', 'load_my_script_for_pmpro' );