The essential ingredient for this magical prank is RainX anti-fog car windshield spray. It's designed to repel the raindrops from a car's windshield.
How? Silicone. Silicone is extremely hydrophobic. It scatters the water that gathers on the mirror, revealing the hidden message.
By the way, do this in advance, leave the house when your 'victim' showers and you'll surely freak them out as to what phantom menace intruded during their shower.