Change checkout URL for promoted wait list registrations (parsed by the [RECIPIENT_WAITLIST_REGISTRATION_URL] shortcode)

wait_list_url_change.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter(
    'FHEE__EED_Wait_Lists__wait_list_checkout_url',
    'my_modify_checkout_link_for_promoted_waitlist_reg',
    10,
    2
);

function my_modify_checkout_link_for_promoted_waitlist_reg(
    $url,
    $registration
) {
    $url = add_query_arg(
        array(
            'e_reg_url_link' => $registration->get_primary_registration()->reg_url_link(),
            'step'           => 'attendee_information',
        ),
        EE_Registry::instance()->CFG->core->reg_page_url()
    );
    return $url;
}