PROC FORECAST is obsolete and is no longer part of the documentation (SAS 9.4 M7).
This is what you get when submitting PROC FORECAST in SAS 9.4 M7 (Maintenance Level 7) :
proc forecast; run;
WARNING: The FORECAST Procedure is obsolete. You may continue to use PROC FORECAST but this is not recommended. For time series forecasting, consider using newer procedures such as the ESM Procedure, the UCM Procedure, or the ARIMA Procedure.
Hence, try PROC ESM !
ESM optimizes the smoothing weights ( and PROC FORECAST does not !! )
Or better, try to find out what Excel is exactly doing and tell us, because there are dozens of possible algorithms that can be applied of course ! Only Exponential Smoothing already has about ten flavors.