So the best way to go about is indeed add that ?sub_confirmation=1 at the end of your url in your about and banner, but you can also add the call to action on all your video descriptions (I do do this) and the best way to do that is using the youtube description bulk updates. I'm not sure how many extra subscribers you'll get from it, but more options are always better and everyone does it. Something like " for more videos subscribre youtubedotcom/yourchannelname?sub_confirmation=1"
View: https://www.youtube.com/watch?v=mU9tahNRVgE
If you already have your url without the code, you can go "find and replace text" to replace the old url with the new url. If you've never included it to your description, you can go "add text to end of description" (better to add to the end of the description as the start of the description is for the proper description of your video)
Very handy tool.