Show username in chat

Hi guys, im making a chat app in C#
I can get it to show time and text but the code im using shows PC username not login username of app
Could someone please help me with the correct bit to put it
Thanks in Advance

            if (textBoxSend.Text != "")
            {
                richTextBoxChat.Text += DateTime.Now.ToString("hh:mm:ss:tt") + " :  " + System.Environment.GetEnvironmentVariable("username") + textBoxSend.Text + Environment.NewLine;
                textBoxSend.Text = "";

            }