Here's a simple IR beam-break project I found online. If you wired this up exactly as-is, the output triggering high should map to the input on the Twine breakout board and achieve what you're looking for. I might have to pick up some parts and make one myself!
I reported elsewhere that I tested a simple photo cell sensor on both the moisture probe and the breakout board. On both of them the photo cell caused the sensor to switch state when light was shining on it. So using that info the job of detecting a beam break sensor should be easy.
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