It's not actually in NewPipe itself as #NewPipe is designed to never pass any information about the user to the services it is downloading the video from.
What I do is hit the share button, copy the URL, then open #Tusky (or your fediverse client of choice), go to the search field, paste in the video URL, and then reply like you're replying a toot.
If you close NewPipe and open the video again (your history makes this easy), it should load your reply as a comment.
Now, if Newpipe could get around the VPN pinch. Starting Saturday, Proton VPN can't display or download YouTube videos. I've had to place it on the VPN bypass list, which really slows down the DL speed. Love Newpipe, hate Google.