Lex v2 Session Attributes

I'm not sure what programming language you are using or this is in a JSON, but if this is in a JSON object and the slot name is MemberNumber, it should be in the following format

"sessionAttributes": {
                //        "Another slot name": "slot_value",
               "MemberNumber": "ABC123"
 },

Thanks for the info - for clarification I'm working within a new Lex bot and drilling down into one intent and the fulfillment section, under Advanced Options. I'm able to reference the slot in a success message where I can say "I got a member number of {MemberNumber}" which works fine and would play the slot value itself, however right below that in the editor where you can set the slot values or session attributes I'm setting [MyAttribute] = {MemberNumber} and the slot value does not get properly substituted, instead I simply get the literal text value of {MemberNumber} instead of what I entered for the slot itself.

Have you tried using {intent.slot} notion instead of just {slot} ?