I'm not quite sure what you mean. Is it something like this?

If so, here's how to do it (or any other indent):
Draw a rectangle using the Create Rectangles and Squares tool. Convert it to a path ( Path > Object to Path ). You will lose the special properties of the Rectangle (F4) tool (you will no longer have the vertical rounding button), but now you can edit the object using the Edit paths by nodes (F2) tool. Select this tool and your rectangle. You will see four nodes. To create a dent, as in the picture, you need more nodes:
Click the top connecting line of the rectangle (step 1 in the image below) and click the Insert new nodes into selected segments button (step 2) (remember that you will see this button only when the Edit paths by nodes tool is Edit paths by nodes )

After that, just drag one node inside the rectangle (step 1 in the image below). To get an experiment with a round shape, for example, using the Make selected nodes symmetric buttons or other buttons. You will notice small handles attached to the node, drag them around.

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