Redmine

Hi,
The howto is not clear enought. I have questions.
There are many ways to setup the link.
In gitlab, you can do it at the admin level through admin/application_settings/integrations/redmine/.
You can do it as well in the project setting.
Where do we set up the link? At the admin level? If yes, what do we put in the field?
Same if it is at a project leve?

In redmine, do we do the setup in the configuration.yml or at a admin/project level in the app?

Thanks for the help.
Regards
Vandman