How to access party identifiers in message mapping

Hi folks, in integration builder I have created a Party, and added an additional Identifier for the vendor number for this Party in SAP backend, e.g.

Agency: SAP | Scheme: ALE#LI#LF | Name: 0000123456

The Party is used as sending Party of an integration flow (PI 7.31 AEX).

In mapping, I need to perform an RFC lookup using the vendor number (as defined above 0000123456) specified for the sending Party.

Anybody has experience with that? Thanks for your help, best regards, Martin