Explicitly stating the purpose of the event in the beginning of the survey is important. This will give the attendees a clear idea of the topic of the survey. Remember to include the product name and logo on all the pages. These are simple methods to build trust.
Ask open-ended questions and include a comments column to discover your potential attendees expectations of the event. A short and simple survey gets more responses. You can then prepare a second-level survey with more questions for people who respond to your first round of questions. Also, your questions should be customized for the event.