External Users and InternetDomainName

I have a question about setting the InternetDomainName on external users
through the AdminObjAPI. The reference is undocumented but if you set
variable = externalUserObject.InternetDomainName it will read the override
parameter that is set in Console One on the Internet Addressing page. When
I try to set this value:

externalUserObject.InternetDomainName = "Some.Value" I get an Ole. Invalid
parameter count in this case.

objNewUser.PrefEMailID = "SomeValue" does work.

I am presently researching ways to create multiple external users inside one
External PO all delivering to different domains. Has anyone found a way to
resolve this issue?

Thanks,
Robert