how to set dynamic payloads in raise fault policy

[#1]

I have a condition which looks for 2 mandatory quesyparams "A" and "B"(say). If any of them is null then I need to raise a fault and set a payload which should only mention the name of queryparam which is null (or both A and B if both are null). What I can do currently is hardcoding the payload with text as- "A and B cannot be null" But if only A is null I need to say- "A cannot b null" How do I dynamically do this, as I cannot use the <condition> there.