Example of how to disable the EE3 Double opt-in emails

example.php

<?php //Please do no include the opening PHP tag if you already have one.

add_filter('event_espresso_mailchimp_list_subscribe_merge_vars', 'tw_ee3_mailchimp_merge_vars', 10, 4);
function tw_ee3_mailchimp_merge_vars( $mergeVars, $eventId, $mailchimpListId, $api) {
    $mergeVars['status_if_new'] = 'subscribed';
    return $mergeVars;
}