The PDF link, page 28, image 12, clearly states it.
Connect Shelly I and O to terminals 20 and 21a; polarity doesn't matter.
However, you may only do this if you also have safety devices such as a light barrier.
"This post does not constitute professional advice. Work on 230V systems may only be carried out by qualified electricians. Implementation is at your own risk."
And just as an aside, if my old eyes aren't deceiving me, that's a Shelly Mini visible in the video? That's not permitted and is dangerous. I hope yours is a Shelly 1 Gen 3 or 4.