YouTube How do you add "Subscribe to my channel" to your cover page?

[selfie style]

I've added my twitter, facebook and instagram icons on my cover page by clicking "Edit links" and adding those links, but how do you do what this guy did:

https://www.youtube.com/channel/UCiQgAN-eT-ktpBoBtJTFv7w

Notice how he also has a clickable "Subscribe to our channel" on his cover page near the other icon links..... How do you do that?

Thanks,
Chirag Shah (Selfie Style)