You have to find the invite code yourself, the link is https://www.hackthebox.eu/invite in order to join the club to learn hacking yourself. The invite code extraction method may change, but this is the current one as of today 25th of Feb 2020.
I am using firefox, so on this https://www.hackthebox.eu/invite, click on web developer on the firefox and choose debugger:
There are a number of functions one particular is interesting
Click on the console, and look for the makeInviteCode() function, the press ctrl + enter key on keyboard to run.
Look for the data field, and also the enctype field which says ROT13. The data field has the ciphertext: “Va beqre gb trarengr gur vaivgr pbqr, znxr n CBFG erdhrfg gb /ncv/vaivgr/trarengr” encrypted by ROT13.
I copy the data into a hackthebox.txt file in my linux machine:
tr command you can translate the data into the actual plaintext.
cat hackthebox.txt | tr "a-zA-Z" "n-za-mN-ZA-M"
So use the uri with an API client such as curl or postman or insomnia and send a post request, you will get a response back with the “code”. The code is encoded in base64, you need to decode it.