The onFormsSuccess handler lets you modify the success message output of a submitted form.

<?php
    $_LW->REGISTERED_APPS['my_app']=[
       'title'=>'My App',
       'handlers'=>['onFormsSuccess'],
    ];

    class LiveWhaleApplicationMyApp {

      public function onFormsSuccess($buffer, $form_id) { 
        global $_LW;
        // use the $form_id to only apply this validation to a certain form, or perhaps
        // track the presence of a field name in $_LW->_POST
        if ($form_id === 6) {
          // modify the output
        }
        return $buffer;
      }

   }
?>