Query parameter to auto-fill private message contents

Recently I've been working on a complete recreation of Tildes Extended, and one of things I've added is a link that automatically takes you to the GitLab issue tracker and auto-fills the issue description with an issue template. That itself is filled out with the browser's info (version, rendering engine, etc) fetched by the extension.

I thought it would be useful if we had this same functionality but for private messages, so that people who want to report something could do that directly through Tildes, instead of having to go through GitLab (which they may not have an account for).

GitLab does this with the issue[title] and issue[description] parameters, like this:

https://gitlab.com/tildes/tildes/issues/new?issue[title]=This%20is%20the%20title&issue[description]=And%20the%20description%20with%20Markdown!%20I%20shouldn't%20submit%20this%20issue%20though.%20%3AP

^{One thing I've learned very quickly though is that you can't use Markdown headers in the URL as then it will be interpreted as an anchor fragment. Gotta use <hx> for that.}