Authentication for private repositories using GitRepoProvider

Hello everyone!

I am trying to configure a BinderHub to have access to a private gitlab repository and I am getting the following error:
Unable to run git ls-remote to get the resolved_ref: fatal: could not read Username for 'https://gitlab.<myserverdomain>': No such device or address.

This only happens for private repos, for public ones on the same server it works. As far as I understood from the documentation, I would need to set the git_credentials in the secret.yaml and enter a username and the access token as the password, but I am having a hard time finding out how it should be formatted, and I found no information on the documentation about that.

Does anyone know how to set this up?

Thank you!