Conversation so far:
What was the Eurovision song contest?
An annual international song competition, with participants representing primarily European countries.
Did ABBA win the Eurovision song contest?
ABBA won the Eurovision Song Contest 1974 at The Dome in Brighton, UK, giving Sweden its first triumph in the contest.

Question: What was the song they contributed?
Self-contained rewrite: What was the song ABBA contributed during the eurovision song contest of 1974?