Stop user from leaving web page, except for when submitting form

To stop the user from leaving my page, I'm using this javascript:

window.onbeforeunload = function()
{
  if($('textarea#usermessage').val() != '')
  {
    return "You have started writing a message.";
  }
};

This shows a message warning the user not to leave the page because they've started writing a message but not submitted. This shows up to save the user from losing this data.

Now, how can I stop this message from showing up when the form is submitted?