@JuLia3870 firstly you need to be a superhost and then you can apply.
I applied to be an ambassador in my area (nearly seven years as a SH since I first started seven years ago) and was told there were too many ambassadors in my location so I couldn't apply - so I had the same experience as @Mike-And-Jane0
It would be good if Airbnb had a badge for ambassadors on their listing so new hosts using their services can understand how long these ambassadors have been hosts and what sort of ratings and reviews they have for their own listings.
From comments on this community it does seem that some ambassadors don't seem to have a very good understanding of how Airbnb works.