1. Draw the required rectangle on the appropriate metal.
2. Draw the circle on the same layer as the rectangle at the required size and in the correct position.
3. Select both the rectangle and the circle.
4. From the menus run the command Edit > Merge > Union Minus Intersection.
5. Done.
Or
1. Draw the required rectangle on the appropriate metal.
2. Draw the circle on a different layer to the rectangle but still at the required size and in the correct position.
3. Select both the rectangle and the circle.
4. From the menus run the command Edit > Merge Logical...
In the dialog box that appears:
5. In the first list select the layer of the rectangle and check the box underneath to Delete Original.
6. In the next list select XOR.
7. In the third list select the layer of the circle and check the box underneath to Delete Original.
8. In the fourth list select the layer of the rectangle again and make sure that the Selected Shapes option is selected.
9. Select OK and it is done.