How do I include causals into an existing forecast model?

Hello,

I am trying to forecast retail sales in an excel based model. I have
built several ARIMA, Smoothing and Winters models in excel to include
trend & seasonality. However I also have information on the causals
such as Advertisements, which I want to include in the model. How do I
include the causals into the model? Is there a way we can add
regression to ARIMA (in excel?). Or may be there is another way to do
it? (Remove Ad factor before forecasting and then add it back??). Has
anyone done this before? Thanks in advance!

-Kiran
.

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