With in ThinkorSwim the Volume Average Indicator is available for use for free. Below I will demonstrate and give instructions on how to add it to your charts. 

Steps 

1.) Open your ThinkorSwim Platform
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2.) Next please go to the "Edit Studies and Strategies" screen by clicking the beaker. 

3.) Once you are in the "Edit Studies and Strategies" screen make sure you are on the studies tab. Type the word "volume" in the studies box. You will see the "VolumeAvg" indicator appear in the menu. Please left click "VolumeAvg" from the studies menu. The click "Add Selected" to add the indicator to your chart.
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4.) Lastly click "apply" and "ok" to place the indicator on your chart. 

Volume Average Indicator in Tradestation
​https://intercom.help/simpler-trading/en/articles/3481522