To be a bit more specific, by default Forecast Studio uses one of the following smoothing models to extend the future input variable values:
• simple
• double
• linear
• damped trend
• seasonal
• Winters method (additive and multiplicative)
The model with the smallest in-sample MAPE is used to forecast the future values of the input.
You can use PROC HPF method = BEST to reproduce the Forecast Studio result if you have the Forecast Server license. If you only have the license for Forecast Studio Desktop version, you will have to run PROC ESM for all the models listed above and pick the model with the smallest in-sample MAPE.
I would suggest feeding the Forecast Studio with future input values (and set the lead = 0) if you do have them available. Often time it is inappropriate to forecast future input variable values if they are not stochastic. For example, promotions, events, etc. Having Forecast Studio to generate forecasts for non-stochastic input variables could distort the shape of the forecasts into the future.