In theory I think you can:

1. put a peer tube video URL in the search box here
2. wait a moment for lemmy to fetch the video page
3. comment on the resulting lemmy page for the video
4. your comment should appear on peertube

However, I just tried it with this video (that instance is running peertube 4.2.0, which is required for some features according to the lemmy release notes) and my comment here has not yet appeared on peertube (nor are the four existing comments on that video appearing on lemmy, nor is the one other video on that channel appearing on the lemmy page for that channel).