do transmission sealer in a bottle work.
All reletive to the location of, and degree of leak. Might help seapage but will not fix a torn or dried seal. The biggest concern with any stop leak ( coolant, engine, or transmission ) is that if you use too much you will stop up the fluid passage-ways where the fluid is supposed to flow.
Personally I would vote for a real fix.
What are you trying to seal? Is the transmission leaking or slipping or what? Also year, make, and model would help.
I had a Trans-X product help an occasional slip in the transmission of my old '72 Dodge Pickup years ago, but that’s a very different beast from a modern automatic.