Spotify should have a default "Quick Playlist" that users could quickly add songs to.
The short key for adding songs to the quicklist should be: B
I keep my quicklist empty for the most time, and when I want to collect songs for something, I use it and afterword I move them to a Playlist and empty out the quicklist.
When I have more playlists I can right click one to "Set as Target Playlist".
Now I can add songs to this Playlist with the B-key on the keyboard.
My source for wanting this, is Lightroom: