I am using Home Designer 2020, and I do not appear to be able to move an opening label as you indicate. I can see that the lower window label is "under" the visible label, but I can't move either.
BUT: I did notice that I could open the window dialog, go to the Label panel, and select "specify label", which will default to the size label. I just entered 2 black lines , which moved the label to a visible area under the lower window size. I can also add a note such as (transom) after the size label.