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 on 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);

You form look 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 you site or about using forms in general.