Mailster forms can be defined and customized via the forms page. You can add additional content via some coding by using the mailster_form filter hook.

This is a basic example of how to add content before and after your forms:

add_filter( 'mailster_form', function( $html, $form_id, $form ) {

    $before = '<p>This is my Text Before</p>';
    $after = '<p>This is my Text After</p>';

    return $before . $html . $after;

}, 10, 3);

Your form looks something like this with this customization:

If you like to target only the Unsubscribe form or the Profile form you can use mailster_unsubscribe_form or mailster_profile_form.

Learn more about how to add custom code to your site or about using forms in general.