Crossovers are not allowed as a fandom request, but what you put in your optional details is up to you. If you would like to prompt a crossover, that is fine. If someone you want to create for prompts a crossover, you may create a work with a crossover in it. There is no way for you to offer crossovers, because you will only match on fandom, character, medium preference, and trick or treat preference. You will not match on optional details or letters, which are the only places where someone can let you know if they would like crossovers.