Dear Robert , Thank you for answering.
Currently our basemap is one of the standard esri basemaps, so the map WKID is 102100 ...
But -- I'm sure there is at least one way which is also relatively easy!
In my WAB (Developer Edition) 2.17 application, instead of using the original WAB's "Coordnate" widget,
I can use the new ESRI's specific CoordinateConversion widget, developed specifically for this very purpose, i.e., to display mouse coordinates "on the fly" oncurrently for any several pre-defined coordinate systems (and the programme can add more).
This widget top description is here (this is same link as above). Click on map jumps to demo page (there I can jump to the sandbox).
In the above widget's top description page it's written: "Additional formats can be created by a developer and made available through the widget.", and below it, in "See also" links list, there are 2 relevant links:
Sample - Coordinate widget (jumps to the demo page mentioned above)
Sample - Add custom coordinate formats - This is most important - it shows by demo + sandbox how to add more coordinate systems (like wkid:2039).
I'll learn this matter, but as all this is new to me, maybe you can help me by showing me how to modify their sandbox code example, in order to add wkid:2039 (TFWKID:108021), so that I'll have something robust to start with?
With appreciation,
Michael