Hello Andreas -
I suspect BOGOF stands for "buy one get one free" and as such it is an indicator about promotional activities.
In my mind you don't want to forecast the values of BOGOF in the first place - as you are really controlling when this promotional activity will occur again in the future, right?
If you provide future values for your independent variable yourself (as you should in this case in my opinion) SAS Forecast Studio will use these values and not try to create a forecast for the independent variable.
Hope this makes sense,
Udo