Write for the scene the player is in
Game music has a different job from a song. It sits under hours of play, so it has to support a moment without demanding attention: a village theme should feel safe, a dungeon should feel tense, a boss should feel urgent. Start your prompt with the scene and the feeling, then list the instruments, and the result matches the moment far better than a bare genre label would.
Describe the player's state as well. Exploration music can be spacious and slow-moving, while a chase or a boss fight needs forward motion, pounding drums and a theme the player can recognise by the second loop. Mentioning that the track is for a game and should repeat gives the generator the right idea about structure.