Draw a rectangle on the edge cuts layer. Sadly this can not be done from within the footprint editor. So your only option is to draw on some other layer and use a text editor to move the drawing over to the edge cuts layer.
Also, the drawing doesn’t specify a suggested or minimum corner radius for the rectangular cuts for the mounting clips. But I see the rectangular cuts are oversized on the 3mm dimension by 1.5mm. Check with your board manufacturer to see what they specify as the standard/minimum inner-corner routing radius. As long as it is 0.75mm or less you should be good. If not, you may need to expand the 4.5mm dimension.