2 small questions

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I need some help with finding and forecasting my ARMA model.
In order to detrend my data I use first differencing. However my data also has some seasonal effect. Should I take care of this before detrending (using first differencing) or after it or can I choose?

Furthermore, I estimate my model with only part of my data (without last year) and after that I use the forecast button to forecast that last year. How is it possible that the serie which contains my forecast also has large values in the period which has not been forecasted?

Thank you in advance