If during registration on SFOR.TRADE exchange you see a cell for entering referral code, but you don't have any referral code to enter, you can continue account registration leaving this cell empty. This will not prevent you from registering in any way.
Note: SFOR.TRADE team cannot add or remove referral codes for your account after you register your account. Thus, if you register on SFOR.TRADE using a referral link of another user or partner of SFOR.TRADE, make sure that his referral code is correctly entered at the moment of registration of your account.