The phrase "as a prank when" is correct and usable in written English.
It can be used to describe an action or event that was done playfully or humorously at a specific time or under certain circumstances.
Example: "He decided to dye his friend's hair bright pink as a prank when he was out of town."
Alternatives: "as a joke when" or "as a trick when".
Facebook thinks I'm playing a prank when I write "No Last Name". Others decided to take it upon themselves to register me as someone else.
It was laughed off as a sweet prank when Christi Lee Wilcox ran onto the field during the first game at Yankee Stadium this season and handed her telephone number to Derek Jeter, the Yankee shortstop.
Tired and irritable, Mr. Daoud said he dismissed the call as a prank, even when the callers warned they had taken his son, Mohammed Ibrahim, and threatened to kill the boy.
I once took part in an abortive burglary as part of a drunken student prank, when not much older than Majid Ahmed.
Some people will use this major news event to doctor and then share photos as a prank as they did when Osama bin Laden was captured.
Did this tall, lean, 50-year-old St . Louisnative with the close-cropped bleached blond hair assume the famed body builder's name as a prank or a fantasy when he came to New York after two years at Swarthmore College in the late 60's?
He was fired when, as a prank, he slipped phony information into an article.
The riot started on Sunday, 13 April 1788, when as a prank a medical student waved a dismembered limb at some boys below a window of the New York Hospital.
"These aren't good enough," he recalled his father saying, even when, as a prank, he handed him shoes that the elder Miskovic had made.
Jan Castaneda says at first she thought her husband had bought a blow-up alligator as a prank but realised it was real when it blinked.
