Create a new GitLab merge request via the REST API, including handling forked-project targets and marking the MR as a draft.

Documented steps

1. Send POST /projects/:id/merge_requests where :id is the numeric project ID or URL-encoded path (e.g. group%2Fproject) of the project containing the source branch.
2. Include the required body params: source_branch (e.g. "feature-x"), target_branch (e.g. "main"), and title (e.g. "Add feature X").
3. curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" --header "Content-Type: application/json" --data '{"source_branch":"feature-x","target_branch":"main","title":"Add feature X"}' "https://gitlab.example.com/api/v4/projects/3/merge_requests"
4. When opening a merge request from a fork, POST against the fork's project id and add target_project_id set to the numeric ID of the upstream project you want to merge into.
5. To open the MR as a draft, prefix the title with "Draft: " (e.g. "Draft: Add feature X") when creating it — there is no boolean draft parameter on the create endpoint; draft/work_in_progress are read-only response fields derived from the title.
6. Optionally set description, assignee_id/assignee_ids, reviewer_ids, labels (comma-separated), remove_source_branch, squash, and milestone_id/milestone in the same request.
7. Docs: https://docs.gitlab.com/api/merge_requests/#create-a-merge-request