Button click to add combo box control

Hello guys, I would like to ask suggestions how to achieve this UI (circle with red color) by using which WPF controls ?

As you see Image 0, there have 2 buttons (plus and minus) and a text box (gap value).

First user will input the gap value, then click "plus" button and combbox UI control will be added.
Besides, as you can see the gap value increase between the 2 combbox UI control.

Thanks.

Image 0


Image 1