I ran across a video making a self-watering seed starter out of a plastic drink bottle. It was on Facebook so getting a link proved problematic (just took me back to the entire page, not that particular post/video). I got to searching around for something similar & found this:
Self-Watering Seed Starter
http://www.homesteadnotes.com/make-self-watering-plant-containers/2/
The idea is the same except for one thing - in the video, a hole was drilled in the bottle cap. A piece of thick yarn was soaked in water, squeezed out & then threaded through the hole, enough to coil around 2-3 times in the bottom of the part that holds the plant. Soil was then put on top of the coiled yarn & that is where seed(s) was planted. The part of the yarn dangling through the hole going the other way went into the water to wick it up into the soil.
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Is this what you saw? There is a video at bottom of page
https://diyallinone.com/2016/12/diy-self-watering-seed-starter-pots/
No, this isn’t the same one, but it’s similar. This one uses a paper towel “wick”. Thanks for the link - with variations of the same idea, you can pick the one(s) you like the best! :-)