If you have a legitimate license (a physical CD case or a dongle) but lost the paper with the validation code, here are the top three solutions currently working.
Some legacy versions of Quark 5.0 for Windows had a known (but undocumented) bypass. When prompted for the validation code, users could sometimes hit "Cancel" three times, or enter a specific string of zeros. Note: This does not work on Mac versions and is not a reliable long-term solution.
, which is a unique string assigned to a specific serial number. Unlike modern software that activates over the internet, version 5.0 requires this static code during the installation process to unlock the software features. How to Find Your Code Original Packaging