just add it at the end of your youtube channel link ?sub_confirmation=1
for example: (my youtube channel link): https://www.youtube.com/channel/UCUExIxwV3Nc1yLxQV74CqXw
after I add the subscription code:
https://www.youtube.com/channel/UCUExIxwV3Nc1yLxQV74CqXw?sub_confirmation=1