I'm from the Netherlands too, and considering signing up for the first time (although it's a bit scary to give my address to a random stranger). I don't know if there's a specific questionnaire or if it's just one big info field, but things that would probably be relevant:

• Allergies and dietary restrictions.
• Even if you have no dietary restrictions, do you prefer chocolate/candy/nuts/...?
• Maybe your favourite scary books/movies.
• Halloween isn't really big here, but do you celebrate it at all? If so, what do you usually do? And if not, maybe you could ask your santa to share some of their Halloween experiences (through gifts, or through a letter)?
• Do you think of Halloween as more of a cartoony, child-friendly holiday with cute ghosts and pumpkins? Or are you more into dark, scary horror stuff?