Conversation so far:
What was ABBA and the Eurovision song contest?
Ulvaeus, Andersson and Stig Anderson believed in the possibilities of using the Eurovision Song Contest as a way to make the music business aware of them as songwriters.
Did ABBA win the Eurovision song contest?
ABBA won the 1974 Eurovision Song Contest on 6 April 1974.

Question: What was the song they contributed?
Self-contained rewrite: What was the song that ABBA contributed?