Well, the referral key asked just after the serial key during the installation of the software is an optional stuff. You don't need to provide that if you don't have one.
By the way, referral key is a way for people who sell the program to get credit for the sale via Microsoft. Referral keys are used in a lot of things, not just MS office. Just for your information.
Hope this one solved your problem. If you have additional doubts, please come back to us.
P.S: If the solution is helpful, please accept the solution.