You can use the Windows.Devices.SerialCommunication name space in your application to create serial communication. Here some code to send a message:
string aqs = SerialDevice.GetDeviceSelector("UART1");
DeviceInformationCollection deviceInfo = await DeviceInformation.FindAllAsync(aqs);
SerialDevice myCom1 = await SerialDevice.FromIdAsync(deviceInfo[0].Id);
myCom1.BaudRate = 9600;
myCom1.DataBits = 8;
myCom1.Parity = SerialParity.None;
myCom1.StopBits = SerialStopBitCount.One;
myCom1.Handshake = SerialHandshake.None;
DataWriter dataWriter = new DataWriter(myCom1.OutputStream);
dataWriter.WriteBytes(<your message>);