Administratively assign user to a role

I have some background processing where a username is available via this statement that passes the username of interest as a variable called "loggedOnUser"

I have been trying something like
Dim UserName As String

            UserName = BuyNowButton1.AdditionalDataItems("loggedOnUser")

            Membership.GetUser("Username")
            Roles.AddUserToRole(UserName, "PaidMember")which does not work.  I think there must be something to setting up the membership bits when the user is not logged on, but am not sure what needs to be done :=(

 
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