The %R indicator can be found near the top of the Insert Analysis Techniques dialog, under the Indicator tab, as outlined in red in the following image:
If you're looking for the Williams %R indicator, then this may not be the indicator you're looking for. Thinkorswim does provide a Williams %R indicator that displays different values than what the TradeStation %R indicator shows, even though the PercentR function call within the %R indicator references Larry Williams. The indicator is typically plotted on a scale with 0 at the top and -100 at the bottom, which is not the case with the TradeStation version. It does appear that way in the Thinkorswim version. In ThinkorSwim, you'll find it under the name, WilliamsPercentR, as shown added to the chart in the following image:
It seems that TradeStation may no longer be providing their App Store as of 2022, so perhaps calling TradeStation support will help locate the correct Williams %R indicator if needed. There are websites offering Larry Williams' indicators for sale.