You do not need to add a reference to User32.dll. It is part of Windows and can be imported in your code without adding a reference. You do this using P/Invoke.
[System.Runtime.InteropServices.DllImport("user32.dll")]
public static extern void SetWindowText(int hWnd, String text);
private void button3_Click(object sender, EventArgs e)
{
IntPtr wHnd = this.Handle;//assuming you are in a C# form application
SetWindowText(wHnd.ToInt32(), "New Window Title");
}
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