Example of adding custom CSS for the Admin area in WordPress. This example hides the Payment dropdown and total fields in Add Member From Admin

my-custom-admin-css.php

<?php
/**
 * This recipe is an example of how to add custom CSS to WordPress Admin
 *
 *
 * 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_custom_admin_css() {
	echo '<style>
	.memberships_page_pmpro-addmember #payment,
	.memberships_page_pmpro-addmember tr.payment,
	.memberships_page_pmpro-addmember label[for=payment],
	.memberships_page_pmpro-addmember #total {
	display:none;
	}
  </style>';
}
add_action( 'admin_head', 'my_custom_admin_css' );