You can either contact the HotelClub and ask for the IHG reservation number or contact IHG and ask for the reservation number.
I'd say your best bet is to contact IHG and give them your name and say you don't have the reservation number. I wouldn't even mention the booking through HotelClub or the HotelClub 6 digit number won't do much for IHG other than help with billing and that whoever you talk to won't likely know how to search for it.