When creating a virtual printer in Linnworks the printer is assigned a machine name and a printer name e.g.
OFFICEMACHINE/ZebraGK420d
In order to create a scannable barcode for the printer visit any barcode creation site such as this one and create a barcode with the value
MACHINENAME\ZebraGK420d
Note the use of just one backslash in the barcode value
You can then print the barcode and affix to your printer to enabling it to be scanned.