Most of the folks on this forum with any real engineering skills and knowledge are based in Europe and Australia. We don't know much about, or have any practical experience of working with, the North American 500 or its AISIN transmission. We also don't have access to any meaningfully useful technical information beyond what can be found on the internet.
With that caveat, I'll try to help you.
AIUI, these transmissions are 'sealed for life' and are not a scheduled maintenance item. Checking the fluid level will likely require both special tools and diagnostics (the exact level will vary depending on fluid temperature) and whilst these tools may be obtainable, you will need to do some serious homework to discover the precise ones you need, and the procedure for using them.
These are, by traditional US car standards, complicated pieces of engineering; we're not in Kansas anymore. It's unlikely fixing this will be as simple as just pouring some fluid into a hole until it comes up to a mark on a stick. If the leakage is from the transmission (and we don't know that yet), then it's likely to need some serious, specialist work to fix it; the cost may be greater than the value of a 2014 500.
Without wishing to offend, from what you've posted thus far, I think you risk getting in out of your depth. My advice is to take the car to a garage with specialist knowledge of a US model Fiat 500 and ask them to diagnose the problem; it may be that means biting the bullet and paying some money to your local Fiat dealer, but I don't think this is something you're going to be able to fix yourself on the cheap.
