Visit the official SketchUp License Manager webpage. Enter your registered email address and serial number (if known) to have the credentials emailed back to you.
If you have purchased a legitimate SketchUp Pro 2019 license, here's how authorization works: Sketchup Pro 2019 License Key And Authorization Number
A secondary code provided alongside the license key that acts as a secure verification password to unlock the software functionality. Visit the official SketchUp License Manager webpage