Read the file as an array of lines: File.ReadAllLines will do that for you from a file, or you can process the QR data directly if that is possible with your hardware.
Then for each line, just use string.Split to break it:
string thisLine = "UserName : michel bondq";
string[] parts = thisLine.Split(':');
if (parts.Length > 1)
{
textBox1.Text = parts[1];
}