It’s an old topic, but you could make use of the auto-fill links. If you want a template one on your forum you could use either the How to create a new personal message via a link with pre-filled information or How to create a new topic link with pre-filled information.
But you can do a similar thing with an [email=] wrap as well:
bugreports@jammydodger.monster
[email=bugreports@jammydodger.monster?subject=Bug%20Report&body=Username:%0A%0D%0A%0DOS:%0A%0D%0A%0DDevice:%0A%0D%0A%0DBrowser:%0A%0D%0A%ODBug%20Details:]bugreports@jammydodger.monster[/email]