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:

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).