I'm beginning to do this myself. Until now I've insisted on a vocal line that I could find lyrics for. But modern producers 'build' complete - mostly electronic - backing tracks that they then need vocals for. That would be the melody, actually.
Ideally, the top-line writer should be the singer as well. Yet if the singer is not a good lyricist you need someone else to complete the song. I'm supposed to do top-line writing for young girls in the future (I am an old man, my voice is deepest bass!). Got to move with the times, I guess...
Bernd