Add icon font to a button

I am trying to add one of the icons to a submit button but when I add the info, it doesn’t work. I have this for my html
<i class="newsletter avia-font-entypo-fontello"></i>
and this for my css

.newsletter:before{ 
  content:'\E75E'; }
}

but it does not seem to work. Am I missing something in either code? It’s for the newsletter dropdown box in my header.

thanks!