“One who prays for the past is uttering a wasted prayer.”

It is obvious that after a person falls down a flight of stairs he can’t pray that he shouldn’t have fallen down the flight of stairs! What he can pray for however is that he should have a speedy recovery.

In your case, once you had the interview or took the exam it is obviously too late to say a prayer requesting that you do well. What you can still pray for is that the people analyzing your interview or exam should have a good impression of your work. Once the envelope is in your hand, you can hope, but there is nothing left to pray for.