It's a url from nsecbunker. There's a live version at nsec.app that he can use. 

 it's an option, but i've had trouble with nsec.app , nak is the most supported for this purpose. https://github.com/fiatjaf/nak 

 How would you bunker sign using nak? 

 

https://image.nostr.build/846af4fd2fe1dd0a92e63bac27ce693a0cafc404306de99dc5c5a6a4c23d3684.png