There are a couple of properties that seem to allow this:
Properties (which is a Dictionaty) AdditionalContext and Context...
I cannot find any guidelines as to which would be most appropriate to use...
I basically need the requestor of the token to provide me with a "External User Id" and the STS I am building will include that as a claim in the response (RSTR).
I would use the .Claims property - this passes in the requested claims.
Add the "external user id" claim here to indicate to the STS that you need this claim in the RSTR.