Amazon Custom – How to create a base ASIN?

Hi all, I am enrolled in the Amazon Custom program and trying to create the “Base ASIN” as instructed by Amazon.

When I go to create the Base ASIN, it is asking me to input a UPC code, but how is this possible?
Wouldn’t the base ASIN act as a Parent ASIN? And Parent ASINs do not need a UPC code

Would I need to use a UPC just to create the base ASIN (almost like a dummy UPC)?

Thanks in advance