On version 2.1.9 and WP 6.+ the add is not working… I noticed others have (had) this issue but an answer is never given on how it was fixed, or if was fixed…
Hey there
Sorry about that, the problem is already reported but indeed its not fixed yet 
Regards
I actually copied the PHP code from that page and pasted it into chatGPT. It told me that it is connected to the widgets which is 4 selections down. I was able to at least use Gutenburg to design something there. It doesn’t follow the theme but hey, I got something custom there.
thanks for sharing