It is fine to ask for unpaid work, but it will narrow down who is available. If you can compensate with food, drink, gift cards, etc., you may mention that under the question of pay.
If there are other circumstances the instructor should know of in advance, mention them at the end of the survey. Common examples include a dress code or the venue always being cold. Please also tell us if they should prepare accommodations for students with learning, sensory, or mobility disabilities.