The lnurl is thr bech32 encoded version of the lnurl pay url used to get the callback. After decoding the hrp, length, validating checksum, can convert to standard byted and get a string which can have domain and username parsed out.

For nostr, users should set the Lightning Address field (aka lightning identity or lud16). The value should look like parcheddebt31@walletofsatoshi.com