You will need to add some client-side code. Ideally it would be a separate JavaScript function with validation etc. But at a very simplest you can modify your Delete button to:
<button type ="submit" class = "btn btn-default" onclick="document.getElementById('action').value = 'deleteMessageFromSimpleForm';" > Delete </button>
Before button submits - it calls handler for click event which changes value of hidden field to new action.