if its an old router with WPS enabled and no security checked then you can run pixiedust on it and crack it within 2 hours locally typically. Pixiedust also has a keygen for WPS Keys and can sometimes generate the proper 8 digit code within seconds.
Otherwise, you need to do as drsnooker said and find a keygen for something of that length or start bruteforcing.
Otherwise, you need to do as drsnooker said and find a keygen for something of that length or start bruteforcing.