Not sure if you've already found the answer to this, but the trick to accessing these context variables once you are in the Action code is to define a session variable with the same name as the context variable (for instance, "slackEmailAddress") and BE SURE to assign that session variable an Initial Value! The initial value can be anything (that matches the type for the session variable). The initial value will be replaced by whatever value your client application passes in with the message context.