The Netherlands international, 21, has agreed to join La Liga champions Barcelona next season, completing a transfer in January for an initial fee of 75 million euros ($84.9 million).
But for now he is focussing on Tuesday's trip to the Santiago Bernabeu and attempting to overhaul a 2-1 deficit from the first leg against Real, who have monopolised the trophy since 2016.
"I spoke to them when I signed the contract and they told me to try to knock out Real Madrid, as that would be great for Barca too," De Jong told Dutch newspaper De Telegraaf.
Ajax's daunting task of turning the tie around has been given a slight boost by the suspension of Real captain Sergio Ramos, who picked up a yellow card late in the game in Amsterdam.
De Jong urged his side to be courageous and believes they can still cause a shock by eliminating Madrid, who have lost their last three games at the Bernabeu and were beaten 1-0 and 3-0 by Barcelona in their previous games.
"We have to play on the attack, put them under pressure and play great football," he added.
"If everyone contributes, we have enough quality to cause a surprise."
(Reporting by Richard Martin; Editing by Christian Radnedge)
