Add search passwords by a hash value
Let users use pre-generated hash values to search. Yeah, I know you calculate hashes of typed passwords on a client side, but some people still prefer not to type their password on 3rd party sites.
You could tweak this github repository to query for your own local text list of passwords: https://github.com/jsrozner/DashlanePwnedPasswordCheck
The code performs encryption on your computer and then sends the query to API with hash only. Your password is never submitted over the internet.
You can, either works
I can certainly understand a reluctance to enter passwords on third party sites, but it becomes a matter of who you trust. People enter passwords online all the time, and HIBP is a secure site. So, as long as they are not storing passwords in the clear, I don't see a reason to be concerned.
Besides, I'm going to assume that the hashes generated client-side may contain a "salt" value and therefore, any hash entered manually by a user (without knowing the salt value) is unlikely to match what's in the HIBP database.