Depends. Will the branch fast-forward into master?
If the answer is ‘yes’ then it doesn’t matter whether you tag it before or after doing the fast-forward merge, because the tagged commit will be the same either way.
If the answer is ‘no’, then you should probably tag it after merging into master (assuming you cut releases from master). In general you want your tags to match your releases (to make it easier to look at the version of the code that was released), so you tag the version in the place you’re making releases from.