af/form/mailchimp/request

<?php

function modify_mailchimp_request( $request, $form, $args ) {
    // The request body is JSON encoded so we must first decode it
    $body = json_decode( $request['body'], true );

    // Set custom merge tag named "NICKNAME" from "nickname" field 
    $body['merge_fields']['NICKNAME'] = af_get_field( 'nickname' );

    // Re-encode the altered body as JSON
    $request['body'] = json_encode( $body, JSON_FORCE_OBJECT );

    return $request;
}
add_filter( 'af/form/mailchimp/request/key=FORM_KEY', 'modify_mailchimp_request', 10, 3 );