Touchback or Down on 1

[KCRef]

I'm a new basketball ref, and my friends think that because I'm a ref, I know football too. They posed this question to me, and I said that I didn't know the answer, but I knew some guys who could tell me the correct answer. Thanks in advance. Here's the situation: Defender intersepts in their own endzone. The intended receiver immediately tackels the defender with the ball, and in the process, he is pushed across the goalline and falls at the 1 yard line. Does he get his forward progress to the 1, or is it a touchback to the 20?