You should look for the PrinterName Property. This property resides in the PrinterSettings Class. The PrinterSettings class is also a property of the PrintDialog. This way you can access the PrinterSettings and change the PrinterName-property.
//Example for GETTING the printername
var pd = new PrintDialog();
var settings = pd.PrinterSettings;
var name = settings.PrinterName
//Example for SETTING the printername
var pd = new PrintDialog();
pd.PrinterSettings.PrinterName = "YOUR_PRINTER_NAME";
Hope this helps.