It should work the same way as loading an outside community, only that the community name is the channel name. These examples should work:

• /c/techlore@neat.tube
• /c/thelinuxexperiment_channel@tilvids.com
• /c/sircmpwn@spacepub.space

I got these from https://lemmy.world/post/1394874

When Lemmy loads these channels, the videos are represented as posts, but cumulated likes and federated comments didn't get carried over. Any comment you make on Lemmy however does get federated back to PeerTube.