Adds recurring donation functionality to the Donations Add On for Paid Memberships Pro: https://www.paidmembershipspro.com/add-ons/donations-add-on/

pmprodon_pmpro_donation_recurring.php

<?php 
/*
* This gist adds recurring functionality to the Donations Add On for Paid Memberships Pro.
* It adds the user-entered 'donation amount' to their recurring billing amount.
*/

//add donation amount to recurring payment amount
function pmprodon_pmpro_donation_recurring( $level ) {
    if ( ! isset( $_REQUEST['donation'] ) ) {
        return $level;
    }

    $donation = preg_replace( '/[^0-9.]/', '', $_REQUEST['donation'] );

    if ( ! empty( $donation ) && $donation > 0 ) {
        $level->billing_amount = $level->billing_amount + $donation;
    }

    return $level;
}
add_filter( 'pmpro_checkout_level', 'pmprodon_pmpro_donation_recurring', 100 ); // priority 100, runs AFTER the plugin's 99