if you want to receive to <your_npub>@npub.cash – you need to prove that its you

You can prove that it's you with a signing extension - like nos2x or alby - on desktop. Then, transfer the cashu from desktop to cashu.me (or other cashu wallet) on mobile via QR. If that sounds a little too janky, don't use npub.cash, just use the minibits app with a minibits address instead.

You can use npub.cash in cashu.me - no need to teansfer, your zaps automatically arrive in your wallet. cashu.me > settings > nostr, then "enable lightning address", done.