When filling the "Configure Repository Browser" in a multibranch pipeline Git section, or in a regular freestyle job, one option is "Fisheye".
However, the URL format is not specified nor hinted. I initially typed http://fisheye.ourdomain.fr, which led to broken links. The correct format was http://fisheye.ourdomain.fr/browse/repoName/

The point is that it is not explicitly documented: Not in the configuration page, not on the plugin page, and I did a bit of googling, to no avail.

I had to open the source code (FisheyeGitRepositoryBrowser.java) to find it, thankfully commented. The source seems to try a validation, maybe it's broken?