add-tag-to-mailchimp-contact.php

<?php
// Note: requires MC4WP plugin to be active and configured, see https://wordpress.org/plugins/mailchimp-for-wp/

// Give users a link to a form, and include "?hfmuc_email={{their_email}}" as a querystring parameter.
// Then make sure the form has a hidden field named "EMAIL" whose value is "{{hfmuc_email}}",
// eg `<input type="hidden" name="EMAIL" placeholder="Your email" required value="{{hfmuc_email}}" />`
// Find your list ID (see https://mailchimp.com/help/find-audience-id/) and put it in $list_id below
// Find the HTML Forms slug (seen when editing the form) and put it in $form_slug
// Add any tags you want to $tags_data

// Now the user will get a link to the form, fill it out (no need to re-enter their email)
// and upon successful submission, those tags will be added to their MailChimp contact.

use \HTML_Forms\Submission;
use HTML_Forms\Form;

// Get the user's email from the query parameters
add_filter(
    'hf_form_html',
    function ($html) {
        $email = isset($_GET['hfmuc_email']) ? $_GET['hfmuc_email'] : '';
        return str_replace('{{hfmuc_email}}', $email, $html);
    }
);
// Adds tags to the given user.
add_action(
    'hf_form_success',
    function ($submission, $form) {
        // Check that it's the right form.
        $form_slug = 'survey-1';
        if($form->slug !== $form_slug){
            return;
        }

        $email_address = $submission->data['EMAIL'];
        $api = mc4wp_get_api_v3();

// aka "audience ID". See https://mailchimp.com/help/find-audience-id/
        $list_id = '32ccd044c3';

// see https://mailchimp.com/developer/reference/lists/list-members/list-member-tags/#read-get_lists_list_id_members_subscriber_hash_tags
        $tags_data = [
            'tags' => [
                [
                    'name'   => 'Test',
                    'status' => 'active',
                ],
            ],
        ];

        $result = $api->update_list_member_tags(
            $list_id,
            $email_address,
            $tags_data
        );
    },
    10,
    2
);