When a user tabs into a textbox all of the text is selected by default.
If the user starts to type something else then the "default text" is deleted and their keystrokes are recorded instead.
Unfortunately that behaviour doesn't seem to work if you use a mouse-click to select the textbox before you start typing. However adding this to the MouseClick event will make it work
private void textBox1_MouseClick(object sender, MouseEventArgs e)
{
((TextBox)sender).SelectAll();
}
If the user moves away from the control (either by using the mouse or tab key) without typing anything in then the default text is left in place, otherwise the entire text is removed when they start typing