I'm afraid that Scheduler doesn't have this feature at the moment. The booking form options are exactly those that are described on the page I mentioned, no more and no less.
You might try to combine a Choice or Survey activity (or similar) with a Scheduler, making the Scheduler visible only when the Survey has been completed (see https://docs.moodle.org/37/en/Activity_completion). This is just a suggestion, you'd need to see whether this works for you.