Fill UM Register forms field using URL GET Method

Hello @crossv

If you are able to write custom code, you can achieve this with the following code

function get_url_param_for_ultimate_member() {
    if (isset($_GET['user_email'])) {
        $email = sanitize_email($_GET['user_email']);
        ?>
        <script type="text/javascript">
            document.addEventListener("DOMContentLoaded", function() {
                var inputField = document.querySelector('input[name="your_field_name"]');
                if (inputField) {
                    inputField.value = "<?php echo esc_js($email); ?>";
                }
            });
        </script>
        <?php
    }
}
add_action('wp_footer', 'get_url_param_for_ultimate_member');

Just be sure to change your_field_name to the name of your user_email field since it can vary.