There is no standard way to do this; WPF does not have the value Fill as defined in WPF.
You can put two path geometries on top of each other. The lower one has a stroke thickness of 0 and has 2 additional points (on the lower edge of the rectangle).
The second vertex will be just the geometry that you now have.
If you need to draw a lot of them, you can create a custom control that does this for you.
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