Fastmail as a Site Specific Browser and mailto: links
 

I've set up Fastmail with a Site Specific Browser (Epichrome), which I really like because it acts as its own app rather than a window in Chrome I have to hunt for.

I can also set that SSB to be the target of mailto: links. BUT, I can't get the Fastmail web interface to open up a new email with the To: field filled in, the way one can with a IMAP client such as Thunderbird or Postbox.

Does anyone know of a way to do that? Or is it hopeless since it is a web browser and not a dedicated app?