I'm hardly an expert, but my understanding was that extracts were made by soaking your extract ingredient in vodka/brandy/whiskey for a few months. If that's the case, the idea would be to rough up some birch bark then soak in a mason jar of alcohol.
I'm a total newb to the whole soda making thing, so perhaps extracts are different in this case.
If you're thinking of something that's more on the sweet side, perhaps you might try Birch Syrup?
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