AFAIK you can't. You are working in the browser and there is no way the server app can access your printers without your intervention. That would be a security risk.
Somebody else correct me if I'm wrong...
From the client side Jan is right. It would be a security risk. For example a Java applet also would need to be signed before allowing automatic printing. If unsigned the user is asked.
From the server side it should be possible, but then in your Mendix app the printer should be known per user. Maybe this can be retrieved from the active directory via LDAP?