In fact, only a few information are required. Your job title aka position (make sense in case is a manager reporting an illegal activity other than a cleaner which might have really small amount of information) the entity involved and if you work in that entity or you just had some sort of relationship.
After all it make sense to have a big set of data to investigate in. If not would be complicated for anyone to understand what's going on. You're still reporting it at the anti corruption authority, so even if you give them more details you should be confident that they'll treat it carefully.
All the personal data (such as name, last name and so on) are optional. And, if you choose to not share your personal data, the form will be considered only if you complied with a level of detail that's enough to do an eventual investigation (if not, since they can't reach you out to ask you or details, would be useless to say "on December 2014 I saw a guy giving 10 euro to another one in the aisle of the department X in the state company Y. But was too dark and I can't tell who they are").