How to get the providerUserKey to create a user if I have 2 membership providers

I need to use 2 different Membership providers in my web application so I have set both of them in the web.config file. I see that, to create a new user with  this approach, I need to issue the following command in code:
Membership.Providers["nameOfProvider"].CreateUser(string UserName, string Password, string email, string passwordQuestion, string passwordAnswer, bool isApproved, object providerUserKey, out MembershipCreateStatus status); What is the "providerUserKey" parameter? How do I get it "before" creating the user?
I found the information in the link below but I just do not get the idea. https://msdn.microsoft.com/en-us/library/system.web.security.membershipuser.provideruserkey(v=vs.110).aspx
Respectfully, Jorge Maldonado