Not sure if my link will work because I already redeemed my free trial so I couldn't get the page back. You can "cancel" your membership and it will only cancel the automatic payment, so make sure to do that right after you sign up. It takes a second. I was charged a dollar to my PayPal account but received a refund like 10 seconds later.
Good explanation of the promotion here https://www.macrumors.com/2017/06/13/youtube-red-free-3-month-trial/ [macrumors.com]
and on the official YouTube Red page.