I don’t have the tag either, but I think you may have to first request addition to the backers2021 group from this page: Mystery Science Discourse 3000 - It's a beta test, we should really just relax.
Make sure you have your backer number from the kickstarter handy.
I just submitted my request, so if you see “Backer” appear next to my name at some point in the near future, it probably worked.