In C#, you don't "order Windows to copy" to the clipboard: instead, you use the
Clipboard class[
^] to copy things onto it:
Clipboard.SetText(myTextBox.Text);
But be very careful: if you start overwriting the clipboard content without the user's knowledge, your software will stand a very, very good chance of being uninstalled with extreme prejudice. Many people use the clipboard a lot, and do not take kindly to you overwriting their data...