On Windows IoT you have to use Windows.Devices.SerialCommunication namespace to access serial ports. You have to have Windows 10 IoT Extension SDK (installer should be bundled with the Windows 10 image file for you board, you have to register there for downloads to be availble) installed and added as reference to be able to access that namespace.
Keep in mind though that if you use Raspberry Pi onboard UART will be inaccessible anyway, as it is used for kernel debugger.