Agreeing with the offer, you walk over to the raccoon boy and gently start stroking the gray and black hairs on his tail, emanating from his behind like a huge fur plume. For a few moments, nothing seems to happen..."what is supposed to ha--", you start to ask, but are interrupted by a sudden buzzing feeling in your skull, soon followed by a shivering feeling running down your spine, and when it reaches the end it feels like your spine is pushing against your back. You cringe at the weird feelings, although it doesn't hurt at all, just feels funny.
Soon you feel something brushing against your skin, as if you have a really hairy brush in your pants, which seem to be filling up with something. This continues for a while, the hairy thing running further and further down your pants...and then it suddenly stops.
"Right, seems like it's done, now we just need to move it out of your pants", says the boy. He reaches at the thing behind you and with a slight tug - which you feel immediately - a large gray and black striped tail floomps out of your pants. After you get comfortable with the idea that you have a real tail, confirmed by feeling the surface - and getting a response from the nerves in your tail - you start practicing movement. In little more than no time at all, you have learned to wave the tail any direction you want.
The boy looks at you joyfully. "Wow, yours is even bigger than mine! I do assume you want to keep wearing pants, right?" You nod affirmatively. "Then we'll have to make a few modifications..." He uses one of his claws to tear a small hole in the back of your pants and helps you get the tail into the hole.
You've got what you wished for: you're now an adult human with a large raccoon tail coming out of the back of your pants, a thought which makes you swish your new appendage merrily.