If you create a tag by e.g.
git tag v1.0
the tag will refer to the most recent commit of the branch you are currently on. You can change branch and create a tag there.
You can also just refer to the other branch while tagging,
git tag v1.0 name_of_other_branch
which will create the tag to the most recent commit of the other branch.
Or you can just put the tag anywhere, no matter which branch, by directly referencing to the SHA1 of some commit
git tag v1.0 <sha1>