When you focus a RadTextBox you will notice the keyboard button that popups next to the focused control. However, for the RadAutoCompleteBox this keyboard button does not show.

Workaround: show it manually on the GotFocus event and hide it on the LostFocus event 

private void radTextBox1_GotFocus(object sender, EventArgs e)
{
    string progFiles = @"C:\Program Files\Common Files\Microsoft Shared\ink";
    string keyboardPath = Path.Combine(progFiles, "TabTip.exe");
    Process.Start(keyboardPath);
}
 
private void radTextBox1_LostFocus(object sender, EventArgs e)
{
    var procs = Process.GetProcessesByName("TabTip");
    if (procs.Length != 0)
        procs[0].Kill();
}