Hi @Ansta0
I'm in South Africa so I don't know much about registration with the NTB and in which cases it would be appropriate or compulsory (I assume you mean the Namibian Tourism Board?).
It would be best to make contact with other Airbnb hosts in Namibia. Airbnb has a resource to help you find your local host clubs (Local Host Clubs - Airbnb Community),
but unfortunately it doesn't seem to list any official host clubs in Namibia (although there certainly are quite a few Airbnb listings in your country).
It may be best to search on social media or to directly approach some local Airbnb hosts, to find a place where you can connect with Airbnb hosts in your area. Another way to approach it would be to contact the Namibian Tourism Board to get their advice on how to approach Airbnb listings.