You don't need a referral code to sign up for Yamgo, but you will need one to become a Club Yamgo member.
When registering for Yamgo, you may have seen one of the fields marked as "Referral Code" on the registration screen. This is a completely optional field, and referral codes are not required in order for you to complete signing up and start earning rewards.
If you intend to join Club Yamgo then you will need a Club Yamgo Referral Code from an existing Club Member.