There are a number of ways to find the serial number of your Link ECU or PDM
On the Packaging
Every Link box has the serial number on one end. Ask your official Link reseller to give you the box when you purchase the product.
Wire-in ECUs
Link wire-in ECUs have a small sticker with the barcode and serial number.
Plug-in ECUs
Link Plug-in ECUs have a small sticker with a QR code and serial number on the board itself.
Razor PDM
Razor PDMs have a small sticker with the barcode and serial number.
In PC Link
If you are a PC Link user, hook up your ECU, and in the software select Help > ECU Information. The serial number will be in the pop-up window.
Ask your Link Reseller
Your Reseller might have a record of your serial number, and more likely if they tuned your ECU.