XI map Message ID from Proxy

Hello,

I need to map the Message ID from a Proxy to my outbound documents. I have one Proxy from R/3 that branches in XI to send out two documents. I want to map the Proxy's message id in both documents.

I have tried the UDF method mentioned in these forums using StreamTransformationConstant(MESSAGE_ID) but this returns the message id of each outgoing document, I need the message id from the original message.

Any ideas?

Thanks,

Matt