To use a Label Designer label as one of the labels on the "Default Labels" screen in Plex. Follow these steps:
1. Set the Application picker to:
/Modules/IntegratedTechnologies/BarCoding/LabelDesigner/Printers/LabelDesignerPrintPage.aspx
2. Set the Querystring textbox to pass the Label Format Key of the desired label (visible on the Label Library).
For example:
Label_Format_Key=1234