iframe subscribe to channel youtube social button

I found a great social button to subscribe to a youtube channel and I want to use that for my youtube apps.
However I get an js error and the problem is I cannot find any info about this iframe button to resolve this. I probally have to add more after channel=
It looks like something from plusone.js (google)

Hope someone can shine a light where to get more info about this social button


cfconsultancy is my channel name so to test it for yourself change to any channel name you like

It definitely is something from apis.google.com with the plusone.js

The code is

But still no documentation at https://developers.google.com . Am I overlooking something here ?

And there is some documentation finally.

Read more about this button here https://developers.google.com/youtube/youtube_subscribe_button