Using the Mailchimp Integration already included in Forminator Pro. This scripts allows us to add one or multiple tags to a Forminator Pro form.

forminator-send-tags-via-form.php

<?php
/*
Name: forminator-send-tags-via-form.php
Description: Sends Mailchimp Tags via Forminator Pro
Source: Konstantinos @ 
URL: https://premium.wpmudev.org/forums/topic/can-i-send-mailchimp-tags-via-form/#post-3624257

Using the Mailchimp Integration already included in Forminator Pro. This scripts allows us to add one or multiple tags to a Forminator Pro form.

install this code in the /mu-plugins folder

In your form have a Hidden Field and add the values as a comma separated list "tag_one,tag_two" ( if you want multiple ), or single if you want 1 tag only "tag_one".

You'll have to replace the "hidden-1" value from $_REQUEST['hidden-1'], into the ID that your hidden field has in the form.

And the tags should be now working 

*/

add_filter('forminator_addon_mailchimp_api_request_data', function ($request_data, $verb, $path) {

    $form_id = $_REQUEST['form_id'];
    
    $tag = sanitize_text_field($_REQUEST['hidden-1']);
    $tags = explode(',', $tag);
    if (is_array($request_data)) {

        if (empty($request_data['fields'])) {
            $request_data['tags'] = $tags;
        }
    }
    return $request_data;
},15,3);