Use a default discount code for the PMPro checkout page.

my_default_pmpro_discount_code.php

<?php
/**
 * Use a default discount code for the PMPro checkout page.
 * Change the DEFAULT code to the one you want to use.
 * You could alter this to check the $pmpro_level global to
 * do this only for certain levels.
 * You could add other logic to exclude existing customers/etc.
 * Add this code to a custom plugin or Code Snippet.
 */
function my_default_pmpro_discount_code() {
    global $discount_code;
    
    if ( empty( $discount_code ) ) {
        $discount_code = 'DEFAULT'; //change this
    }
}
add_action( 'pmpro_checkout_after_parameters_set', 'my_default_pmpro_discount_code' );