There are a few issues with this code, but the biggest one is that you've created a clone and should be modifying the clone (pOutput) exclusively. However, you're still calling SetValueAt() against a segment pulled from pRequest.

The above notwithstanding, you're still sending the original unchanged message object (pRequest) to the target operation with the SendRequestAsync() call.

Fundamentally, you should:

1. Create a clone of the request object pRequest (i.e. pOutput)
2. Modify the clone, which is a mutable copy of the original message
3. Send the modified clone (pOutput) to the target operation