Set target_branch to the ref branch when creating MR from issue

app/services/merge_requests/create_from_issue_service.rb

@@ -54,6 +54,7 @@ module MergeRequests
         source_project_id: project.id,
         source_branch: branch_name,
         target_project_id: project.id,
+        target_branch: ref,
         milestone_id: issue.milestone_id
       }
     end

changelogs/unreleased/41727-target-branch-name.yml

+---
+title: Set target_branch to the ref branch when creating MR from issue
+merge_request:
+author:
+type: fixed

spec/services/merge_requests/create_from_issue_service_spec.rb

@@ -112,5 +112,24 @@ describe MergeRequests::CreateFromIssueService do
 
       expect(result[:merge_request].assignee).to eq(user)
     end
+
+    context 'when ref branch is set' do
+      subject { described_class.new(project, user, issue_iid: issue.iid, ref: 'feature').execute }
+
+      it 'sets the merge request source branch to the new issue branch' do
+        expect(subject[:merge_request].source_branch).to eq(issue.to_branch_name)
+      end
+
+      it 'sets the merge request target branch to the ref branch' do
+        expect(subject[:merge_request].target_branch).to eq('feature')
+      end
+
+      context 'when ref branch does not exist' do
+        it 'does not create a merge request' do
+          expect { described_class.new(project, user, issue_iid: issue.iid, ref: 'nobr').execute }
+            .not_to change { project.merge_requests.count }
+        end
+      end
+    end
   end
 end