Get all your serial ports by calling
SerialPort.GetPortNames().
Select the port you want, but
SerialPort won't help with that. You need to either let user select a port, or send something to every port and see which one responds in the way you expect it to. Be prepared for some exceptions on ports that are in use by other software.
Store selected port by its name in
Properties.Settings.Default.RS232ConnectionName. That way, you can change it from within your software and get it again
OnLoad.