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This should work if the external resource is known to your system John - unless you have it locked down in some way.
My organization has very strict security controls because of the nature of our work. Getting someone added as a "guest" is a very high lift. Ideally it would be possible to simply create a "generic" or "material" resource that I can custom name and add to the Planner plan without needing to add a full profile of any kind.