In Social Studio › Instant Post, the YouTube card’s More YouTube options now has Playlists: tick up to 10 of the channel’s playlists for the video. Each shows how many videos it holds and whether it is Public, Unlisted or Private; a search appears when there are more than 8. Nothing is ticked at the start.
With One post per video, every row has Add to playlists (Playlists · N once chosen). The video goes into its playlists after the upload — a scheduled one at its time. If YouTube refuses a playlist, the video stays up and the row says «Posted, but YouTube didn't add it to «…»: …».
The phone app has the same Playlists list.