Hard to tell by the picture, but it doesn´t look like an actual knot there. Looks like the thin piece that is for the back is just run through the folded-over front piece. The problem is inside MD, the mesh of one piece will tend to ´pull through" the mesh of the other at the sort of tension needed to hold up the dress. So we fake it.
Check the image. The key is the small purple square. That is the base for the whole construction. Give it a very stiff setting (I used Leather Belt) so it wouldn´t collapse or twist.
After that, just sew the skirt and attach the bodice pieces. (You will need to adjust the size of the various rectangles to suit your avatar´s proportions, of course. This was just a mock-up test.