The letter reflects on different moments the pair have shared together.
"I just want you to know that you're the most amazing man I've ever met ... You've raised me my whole life, from putting my hair in tight Princess Leia buns, forging my signature like in fifth grade, jamming out to alternative music, going to Warped Tour and even our first rock concert together, leading up to us laughing at cats wearing leggings," the letter read in part.
"I know we are so weird together, but that’s what makes you, you, and I cannot imagine not having you in my life. I’m so grateful to be able to call you Dad."
Farrell opens the box after reading the letter to find a Superman onesie and two boxes of candy.
Underneath those treats, he sees his main present -- adoption papers -- and embraces Knight while wiping away tears.
"I finally asked the man that raised me to adopt me," Knight captioned the video. "(I) love you with all my heart and can't wait to be a Farrell."
The video, which was posted Saturday, has over 27 million views.
Watch the video below.
So everyone has been waiting for this but I finally asked the man that raised me to adopt me ❤️ love you with all my heart and can't wait to be a FARRELL Ryan Farrell