Using element of message in condition editor in BPM

Hello! I have an abstract interface "si_AA_mdm_file" using message type "MDMBusinessPartner". In this message type I have an element "ExternalPartnerNumber":

<?xml version="1.0" encoding="UTF-8"?>

<MDMBusinessPartner remoteSystem="" schemeVersionID="">

<Record>

<ExternalPartnerNumber>222</ExternalPartnerNumber>

...

</Record>

</MDMBusinessPartner>

If the message has this element, I need to use one SwitchBranch, if not - other Switch Branch.

How can I do this?