At the top of the shader dialog are four buttons. The second one will allow you to draw a rectangle on the image of the band in the preview window. The appropriate layer and dimensions will be created automatically in the shader tree. Choose Multi channel as the shader for the rectangle. BTW sometimes the rectangle on the band will look kind of funky. This seems to be a bug in the display. The rectangle is usually ok. The first button at the top (with the arrow) will allow some editing of the size and placement of the rectangle or you can play with the numbers in the top, height, left and width boxes.