Adds a hint under the "Username" field at checkout.

add_username_hint_at_checkout.php

<?php   // do not copy this line.
/**
 * This recipe Adds a placeholder hint inside the "Username" field at checkout.
 * 
 * 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_checkout_add_username_placeholder_hint() {
	?>
	<script>
		jQuery(document).ready(function($) {
			$("#username").attr("placeholder", "Choose wisely — this can't be changed later.");
		});
	</script>
	<?php
}
add_action( 'pmpro_checkout_after_username', 'my_pmpro_checkout_add_username_placeholder_hint' );