Order Management Defaulting Rules for Header DFF

Hello,

While importing order (it is ISO generated from IR) , I need to default a value in Header DFF attribute1 (Additional Header Information).

How do I use defaulting rule for this purpose ? DFF are not shown in the defaulting rule form.

I was thinking of creating a defaulting rule for one of the unused field like FOB point, at header level and then define source of the rule to be API and update the cache attribute as below.

ont_header_def_hdlr.g_record.attribute1 := 'XYZ';

Is above allowed ? I do not want to go the route of trigger on IFACE table