Just draw a rectangle with a rounded rectangle tool. Then use the pointer tool and select the created rectangle. You will see four diamonds. If you drag one of the diamonds, you will see that you can change the four corners at the same time. If you press the alt key (option) while dragging, you can change one separate corner of the rectangle.
Another possibility would be to create two rectangles. One with rounded corners and one without. Then use the put command. The way.
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