To generate a schedule, participants always need to have been added to your tournament first. If you're still waiting on registrations, you can already get started by working with placeholder teams.
Once you receive actual registrations, simply change the name of the placeholder team to the name of the real team. Everywhere that team was already scheduled, the name is automatically updated as well. So you don't need to recreate the schedule.
If you let online registrations run through your registration page, new teams are added separately. You can link such a registered team to one of your placeholder teams using the Vervang ploeg (replace team) button. This automatically makes the real team take over the place and schedule of the placeholder team.
Tip: don't set a maximum number of registrations if you've already created placeholder teams, since this would make your tournament appear full while there are no actual registrations yet.
This way, you can already prepare your entire tournament schedule, including groups, match dates and fields, even before registrations are finalized.