Welcome Wolfgang!
There are many ways to achieve this.
I would use the parallel line tool. On the top is the tool option bar, where you can enter the distance for the parallel line, when the tool is active. In this case, the distance is the height of the new rectangle.
Then, with snap on endpoints active, you can draw the left and right vertical lines using the 2-points line tool.
Or, if you need to have a closed rectangle, start with the parallel line too, but then use the rectangle tool with snap on endpoints active. Use the existing rectangle for the bottom left point and the right end of the parallel line as top right point of the new rectangle. At last, you can delete the parallel line again.
Armin
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