Add the HTML tag where {printerName} is the name of the printer to set as default.
Example
<PARAM name="user.LocalClientPrinters" value="Laser">
<PARAM name="user.Laser.port" value="\UNC-SERVERUNC-PRINTER">
<PARAM name="user.Laser.driver" value="HP LaserJet 5/5M PostScript">
<PARAM name="user.Laser.comment" value="WFCDefault">