Can't add song to someone else's playlist, using Web Player

[Taimatsu]

I'm a Premium user, in the UK, using Web Player on Chrome for Windows 10.

There's a playlist a friend has shared with me, which I can contribute songs to using the iOS app. But on Web Player, I can't add songs to it - when I pick a song and choose 'Add to Playlist', it doesn't show me the ones shared with me by someone else. 

How can I add songs to my friend's playlist using Web Player? It's really frustrating.