How to customize a merge request message on Gitlab?

Globally, automatically - I don't think so. As I see, it's hard-coded:

message = [
  "Merge branch '#{source_branch}' into '#{target_branch}'",
  title
]

if !include_description && closes_issues_references.present?
  message << "Closes #{closes_issues_references.to_sentence}"
end

message << "#{description}" if include_description && description.present?
message << "See merge request #{to_reference}"

message.join("\n\n")

You can override message for any merge request manually:

enter image description here

It's also possible if you create merge request with API. It requires your time but you can build some mechanism that fetches all data with API and set it as a description (but you must ensure all is available with API, issueDescription and so on).

Tags:

Gitlab