"Add Release" says label under "Edit Note" tab the label info will be added to a release (but actually won't be), when label field has text in it.

DIY:
1) start adding a release ("Editing → Add Release")
2) Enter text of your choosing into the label field. Do not search for a label!
2.5) Add enough data for the release, that you can proceed to the "Edit Note" tab (release title, artist, tracklist, etc)
3) On "Edit note" tab note that under "Add release label" it shows the label name you entered, fake or not. Also note that "Add Missing Enities" tab was skipped. No error or warning is given.
4) Click "Enter Edit". Note that the will not be added to the release. If you entered a catalog number, that will be stored.