You'll need to connect your YouTube account credentials and supply the ID of a given Playlist on YouTube.
The playlist should be set such that "Add new videos to top of playlist" is off, it may work both ways but seemed to work better when testing when that was off.

You'll also need your ToWatchList API Key available at ToWatchList.com/Tools.

A final tip, if you'd like to import all your subscriptions into a playlist to add to ToWatchList, check out this GitHub Project: auto-youtube-subscription-playlist-2. It can also do neat things like combine other playlists etc into one playlist (which you can then import to ToWatchList).