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:
This idea has been submitted more than 1 year ago but unfortunately hasn't gained enough kudos during that time (10 kudos/year). In order to keep an overview of the active ideas in this forum, this idea will be closed for now. Note that this does not mean the idea has been declined by Spotify.
Please feel free to re-post this idea if you still feel you want this implemented. With a little change in presentation, maybe this time it will gain the necessary support from the community.