You should ask this question on your tourist board, it has nothing to do with Airbnb or Bookingcom, or any other agency, they don't have rules about it.
For example here in Croatia hosts are informed what to do if their guest gets sick or positive during the stay.
These are our options:
1- if a guest arrived by car he has to go home
2- if the guest arrived by plane then we can:
- allow them to stay (if our unit is available and if it is not shared by the host or other guests) and charge the guest for their extended stay
- or we can notify the authorities to relocate him in the quarantine.
Here is the article so you can try to google-translate it
https://www.index.hr/vijesti/clanak/sto-sa-zarazenim-turistima-moraju-imati-pcr-test-preporucuje-se-...