People are making this too complicated.
You have a broken spoke. Presumably it's broken at the hub end. Unscrew it from the nipple by spinning it in your fingers, or holding it with pliers as you turn the nipple with a spoke wrench
Spokes are measured from the inside of the J-bend to the end. Your spoke broke right at the J-bend, so you need to eyeball where the inside was by what's left. Worst case you'll be off by 1mm, which is OK.
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