You can try Temper first once without needing a VAT-ID. During the trial period, you can check whether entrepreneurship is something for you. After you have tried it once, you need to request for a VAT-ID to become a FreeFlexer.
Note: Are you already a freelancer with a VAT-ID? Then add your VAT-ID to your profile and immediately try out the platform as a FreeFlexer.