Automatic cars usually have two dipsticks, one for the engine and one for the transmission - for these it's usual to use the transmission dipstick tube as the filler for it.

Make sure you're checking the transmission fluid level correctly - it needs to be done with the car warmed up, engine running, transmission in Park - wipe the dipstick then re-dip to check the level.

An increasing number of automatic transmissions have no dipstick so can only be level checked, topped up or refilled through a plug on the casing.

Very worrying though, is the possibility that you've already added transmission fluid through the engine oil filler - that'll do the engine no good at all so a complete engine oil change including the filter is urgently needed if this is the case.