It looks like you have done everything right, so I'm not sure why the custom window won't appear.
However, the DarqUI Threat window (Ctrl+T) also has auto-attack bars that you can turn on/off, and you can optionally assign a sound to assist with autoattack timing. Also, you can set the default auto-attack bars to show by going to Options (Alt+O) >> User Interface >> Game Windows >> Show the autoattack timing bar. Both of these other options should work very similarly to samejima's window.
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