What is best way to tell a girl that i like her ass smell?

tell her to take an enema

Cosmo seems to think this is the same as making a request in a retail environment. While it looks good on paper, it would be awkward in reality.

Just put her butt in the air doggy style and eat her poon-snap out from behind. You already know where your nose will end up. Right up that sodomy hole.

Then if she opposes, just say in a raised voice, "Look, I am a REAL squat-hole noser, just like people working in an office."

I do not know what swamp-ass means, it sounds gross.

Just flip her over and tongue punch that fartbox

Just say "Please may I sniff your stinky asshole?"

An excellent question. First you have to approach the topic cautiously. Tell her something like you're going to have to excuse yourself to the bathroom soon because you have a bad case of swamp-ass. Then, now that you're on topic, mention that you hope the swamp-ass doesn't cause any bad ass smell. At this point it's time to zero in and make your move. This is when you tell her that, come to think of it, you actually quite like her ass smell. From this point on you're home free.

Okay, this title made me chuckle. Seriously though, you gotta amp up your game.