Create merge request for *new* target branch?

I don’t see an option in GitLab to create a merge request for a fork that specifies a new branch. Granted, it wouldn’t be called a “merge request”, perhaps a better term would be “pull request”. To describe the workflow: as a contributor to a project, I would fork the project, check out a new feature branch, do some work, commit, push, then I’d like to send the new feature branch to the original owner. It looks like the only option from GitLab is to send the merge request to the project’s default branch (usually master) which is not all what either of want. So how do I accomplish this?

you can only use existing branch in MR (same is the case for PR). Creating a MR/PR from your fork to original project is the standard workflow. There is no way to “send a branch to another project”. You can ask owner of the project to create a branch and merge it there or give him permissions to your fork so he can pull/push to the original project.