How to trigger Mashup button when you press Enter in textbox?

I want to bind mash button with keyboard 'Enter' key in-addition to mouse click. I have added below code in snippet of service but it doesn't work

document.onkeydown = function (e) {
  e = e || window.event;
  switch (e.which || e.keyCode) {
   case 13 : //Your Code Here (13 is ascii code for 'ENTER')
   break;
  }
}

Could you please help on this?