how do i submit pull request to someone’s plugin

I would like to submit a pull request to someone else’s plugin. I would prefer to do this using git but can lookup the svn command if necessary. I have a wordpress.com login. What’s the url for the repo that contains this file: https://plugins.trac.wordpress.org/browser/forms-gutenberg/trunk/readme.txt

I’m sure there is a getting started doc somewhere I should refer to but I can’t find it. They all seem to assume that there is one person doing a plugin or theme and creating and submitting to their own project.

The page I need help with: [log in to see the link]