EL Expression to make Misc Inventory transaction DFF conditionally mandatory based on Source

Summary:

Hi, can anyone help with the EL expression required to make a DFF on Miscellaneous Invenotry transaction field conditionally mandatory based on Source through sandbox page composer?

Content (required):

The requirement is to make the DFF conditionally mandatory based on Source but we are unable to find the relevant binding parameter.

Note that we have achieved to make the DFF conditionally mandatory based on Item description and Transaction type.

Since the Source is a KFF, we are unable to fetch it as binding parameter.

There is Segment1 opened with name "SAN Type" and then there is LOV attached to it which list down Sources.