Signal maps are just holders for signal behaviors and don’t hold values by themselves.
You need to get the signal behavior (vcSignal) that is connected to the signal map port you want and then use the vcSignal.Signal() method to trigger the signal with given value.
PS: The signal behaviors used with signal maps might not be visible in the Modeling tab, but are accessible through API. The “Signals” editor UI and Connectivity feature create such hidden signals when you connect something to a signal map port that doesn’t already have a signal.
PPS: This is a related topic: Data Transfer PLC to Inputs - #4 by Mathias