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The credentials are the url of the server and an private api key. The server has to be reachable from the confluence server, in order to make the macros work properly.
The private api key can be generated on the GitLab server (there it is called personal access token). You can find it under Profile Settings (click on your avatar icon on the right) → Access Tokens.
At this place you can generate a unique access token for the GitLab Api Connector plugin. Be aware that the personal access token is only shown once after the creation.
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