PEG Ratio

First we need a clear definition of what the PEG ratio is and how it is computed:

https://www.investopedia.com/terms/p/pegratio.asp

From this we find that we need the earnings per share and the analyst's expected earnings growth rate.

Here is a full list of language elements that allow us to display fundamental data for individual stocks:

https://tlc.thinkorswim.com/center/reference/thinkScript/Functions/Stock-Fundamentals

From the list of available metrics we find earnings per share but there is nothing in this list that shows analyst's expected earnings growth. To be clear, earnings per share is historical in nature. While analyst's expected earnings growth has no historical context because it is always based a projection that is based on analyst's expectations. There are numerous sources for this data and Thinkorswim does not provide access to any of them from within the language of Thinkorswim.

In the current version of Thinkorswim we do not have any way to compute the PEG ratio. This sort of metric would need to be hard coded into the platform by the developers of Thinkorswim. So if this is something you feel is very important to making trading decisions I suggest users submit a feature request to TD Ameritrade support requesting this metric be added as a built-in column that can be added to the watchlist.