How do I create a merge request for a project?
As said in Issue forks &amp; merge requests drupal.org only implements (so far) issue forks and merge requests related to those issue forks. Issue forks are still in development and enabled only on those projects that opted-in, a list of which is available in Opt-in core issues into the Drupal.org Issue Forks and Merge Requests Beta: The list doesn't include the projects maintained from the Drupal Association, for which issue forks have been directly enabled from a Drupal Association employee.
The plan is making them available on all projects on November 10, 2020.
When issue forks are enabled, an issue opened for that project contains a button as in the following screenshot, taken from Show commit activity for issue fork branches without merge requests.



When an issue fork has been created, there will be a different button, as in the following screenshot.



I didn't watch at the code used on drupal.org, but I expect the code checks users have the Git user role on drupal.org, which is given to the users who accepts the Git access agreement, or a Git account on Drupal.org, before allowing them to create an issue fork or obtain push access on an issue fork.



Merge requests aren't tested on DrupalCI, until Test merge requests made in GitLab with DrupalCI is marked as fixed. That could happen before, or after, issue forks are enabled on every project.
As for today, December 24, 2020, issue forks and merge requests have been enabled on every project hosted on drupal.org. Merge requests are tested by the DrupalCI instance running for drupal.org.