Hi Karen
We have the same issue here in Palm Springs: an 11.5% occupancy tax on short-term rentals. Since there's no way to add it as an additional expense ( like cleaning fee), Airbnb advised me of my options when I contacted them for advice:
1. Build the tax into your nightly rate (downside: your nightly rate will be inflated in search results)
2. Collect it when guests check in (okay, but sounds dicey and not a good customer experience)
3. Send the guest a Special Offer Rate with nightly rate, cleaning and tax all bundled (but make sure you spell this out in your listing)
We do the third option and while it requires more steps and sometimes additional explanation to the guest, it by and large works for us.
Hope that helps!