Pulled this from Stack Overflow... this URL seems to do the trick
We're integrating with Slack and have a bot letting us know when new emails to our Google Groups come in. I was looking for this same thing to add a hyperlink to the Slack notification that allows us to quickly reply to the sender via Gmail (instead of mailto trying to use Outlook or another default mail client).
Haven't found a way to pre-fill the "To" field in the nice Gmail pop-up composer though... only through this full screen option.
Stack overflow source