see where the leak occurs

if it is gasket, then change it.  If it is from the seal between tranny and engine, donate the car.  You can add the ATF from the dip stick tube (yes that is the place to add the fluid).  But seems you have to do something to stop the leak.  DO NOT add additive or other compound to fix the leak.  It will not work for tranny or will damage the tranny.