The “extremely valuable” 800-word work was penned on both sides of the A5 card for a charity auction nine years ago and sold for £25,000 at Sotheby’s.
The manuscript and a quantity of jewellery were stolen from a property in Howard Road, Kings Heath, Birmingham, during a break-in between April 13 and 24.
West Midlands Police have issued a plea for information in the hope that “the appeal goes far and wide among Harry Potter fans throughout the world.”
Investigating officer Pc Paul Jauncey said: “The only people who will buy this unique piece are true Harry Potter fans.
“We are appealing to anyone who sees, or is offered this item for sale, to contact police.”
The prequel was among several works donated by authors, including Sebastian Faulks and Doris Lessing, to an auction held in 2008 to raise funds for English PEN, which promotes understanding through literature, and Dyslexia Action.
Reportedly set three years before Harry Potter’s birth, the prequel features the schoolboy wizard’s father, James Potter, and godfather, Sirius Black, when they were teenagers.
They are confronted by two “muggle” policemen after a high-speed motorbike chase but manage to escape on broomsticks.
Anyone who knows the whereabouts of the postcard, or who has information about the break-in, is urged to contact West Midlands Police on 101 or Crimestoppers anonymously on 0800 555 111.