Where to configure the "issues_tracker_id"?

Perhaps someone can give me a hint regarding the project’s issues_tracker_id? We’re now luckily migrating our SCM to GitLab but are using Redmine for the issues. The new redmine service integration does a good job once manually configured. There is also the possibility to use the „:issues_tracker_id“ placeholder, but I cannot find any way to set the value for this field using the UI. What am I missing?