Source code is here: https://pastebin.com/GS6m3dJP
I put it together using various examples, howtos and check_mk documentation.
I used this article a lot: https://www.steinkogler.org/2016/08/21/check-mk-write-your-own-active-check/
The check needs to download a web page, parse the data out of it and then evaluate the data.