The easiest way to get a .bnb domain name, is through Space ID.
Just search for your domain name, register it and then set is as primary domain for your wallet. After a short time your .bnb address should start showing up on BSCscan.
What is BNB Domain Service?
The BNB Domain Name Service simplifies the search and registration process for Binance Chain users, allowing them to register a BNB Domain Name with only one click and less than $10 on average in less than 1 minute. It is built on the foundation of ENS but provides a better user experience.