The easiest way is to use a wall for your countertop.

First tilt your sink so the top points forward. Export the sink. Import the sink and check "Door or window" in the import dialog.

Create a wall as if you have your counter top standing on its front side. Insert your import "window-sink" in the wall and adjust its position and elevation. It will create a hole in the wall the size of your sink.

Only select the wall and export it as countertop.obj. Import it and tilt it back so it is horizontal again. Export/import and you have your countertop with a hole for your sink.

A little tip: If your sink has a small border you should make the sink a little smaller before export and import as a window. This will make the hole a little smaller so it stays under the border of your original sink.

Check the manuals page on dodecagon.nl for a manual on how to change material names. A material "Top" looks better than "wall_1_1" if you want to set the color or texture of your countertop.