page1
The dotted line needs to be an additional object in this case.
To be able to position it precisely relative to the points oif the polygon, you need to enable Snap to Grid and/or Snap to Object Points depending on your specifiuc intentions.
You can’t connect the end points of the additional line to specific poiunts of the polgon persistently, but only the line as a whole to the polygon as a whole. If you need to edit the positions of individual points later, you may need to do this for the additional line additionally.