You can't do it directly.  The browsers are designed to call desktop mail clients rather than webmail.  Each browser operates differently, so a solution for one won't work for others.  I think it would be a good idea for ST to write the necessary extensions to make this work for the major browsers, like Gmail does.

 

You need to do some searching for mailto extensions for the browser of your choice,  Here's a link to that search for Firefox:

 

https://addons.mozilla.org/en-US/firefox/search/?q=mailto

 

If you look at the extension named MailtoWebmails, you'll see that the author offers to add support for different webmail on request.  I have no idea if he currently supports SmarterMail or not, but perhaps you could request it.