Where I labeled the word "Here" is where I am having trouble. I need to
form a strip of concrete overhang along the top edge of the window, over
the wood framing. I need to do this because there are three other
equivalent size existing windows that looks like that along that wall so I
need to match it.

To "match" the existing..what is the surface coating above the existing
windows..Painted concrete ? Stucco ?
Can you use foam 'block' material they use for stucco bump outs: attach,
then texture (stucco) and paint to match ?