If your company was a volume license customer, you might still have a record with Claris (the current developer of FileMaker). They may be able to retrieve your license certificate if you can provide proof of purchase or other identifying information, though support for such an old version is extremely limited.
FileMaker Pro 6 requires a specific alphanumeric code (license key) during installation. This code validates the purchase and enables all features, including peer-to-peer sharing. Locating Your Installation Key filemaker pro 6 installation code hot
While a single user is unlikely to be pursued for a 20-year-old software, the legal risk, combined with the security threats, is a major deterrent. If your company was a volume license customer,