I just managed to get through this! You can register on their official website (I'm not adding the link as it could trigger spam alert )

In the passport / ID section, you need to add one alphabet at the front (I chose A), followed by my 9-digit UK passport number. My guess is, you don't really need to put your real number as the initial alphabet that we put at the front isn't accurate at the first place!
Then you choose country of citizenship and residence of yours.
The password combination must be as per the instruction, but the length is in fact limited at 8 digit max, not more.