@Eliza-Jane0 A suggested place to enter that information (per some AirBnB Help note I recall from awhile back) is in the "Other things to note" field in your listing's Description. However, depending on the format of your locale's license value, AirBnB may actually censor the number if it looks too much like a phone number. That's actually what happens here in Austin.) So what we do is let that censored piece of text remain, then we add (for the benefit of the city inspectors) text saying "Hmm... Apparently AirBnB suppresses the number because they interpret it as a phone number. If you're a city official, please contact us if you want to verify the license number."
I'm sure there are other ways to handle the situation and, again, your particular municipality's license number might not get blocked.