The question is in the title.
It looks like its Newznab based so it already is.
I don’t have access to Newznab. New registrations by invite only.
However I do have access to NZBNDX but it does not appear in the list of Indexers.
I currently use OZnzb and Omgwtfnzbs so perhaps NZBNDX may be superfluous.
Newznab isn’t an indexer, its a generic indexer type. Click the big button that says Newznab and add that indexer. OZnzb is a Newznab indexer and it happens to be one thats listed in Sonarr.