iframe subscribe to channel youtube social button

Envato Customers Looking For
1

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

Example

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

2

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 ?

3

And there is some documentation finally.

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