If Airbnb already has arrangements with your locale, it will automatically be calculated with the booking and remitted on your behalf. There is not currently a way you can add it as a separate line item at this point. So you can either gross up your listing to include the tax or clearly spell out in your listing how you will collect the tax (some people ask for it in cash upon arrival, some people will send a request through the Resolution Center after the guest books, etc).