Hm, in that case I would advise you to offer the parent tag. AO3 literally won't accept a non-canonical tag in your sign up, so the best option might be to default to the parent tag for the purposes of the sign up form. Whoever you might be matched with that requests that fandom will have to do the same thing.

What's the name of the podcast? I can put up a short note here on the DW to notify any potential requesters that they'll need to use the Shadowrun RPG tag.