When you add a circle or box graphic element to a schematic, the Properties window provides a FillStyle option so that you can choose Hollow, Solid, etc.
However, when you add a multi-segment line (polyline) by adding a line and clicking the spacebar for each vertex, there is no way to fill the shape.
Actually, there is a way, but you need to to into Setup -> Settings, change the default fillstyle for lines to Solid, then re-draw the shape.
Attached is a script that lets you select the polyline shape, right-click, and choose "Fill Line" or "Hollow Line" to toggle the fillstyle between solid and hollow.
Load the script on startup by adding an entry to the [viewdraw] section of a scripts.ini file: