This is my script:
# Projecting levels value of x1 for 3 periods ahead, 2016-2018
Dataset addobs 3
scalar x1_GROWTH= 5 # Annual Percentage Growth
genr x1["2016"]= (1+x1_GROWTH/100)*x1["2015"]
genr x1[2017"]= (1+x1_GROWTH/100)*x1["2016"]
genr x1["2018"]= (1+x1_GROWTH/100)*x1["2017"]
fcast 2016 2018 variable_name
The last x1 value is for 2015 and these generated x1 values are then put into an existing series x1.
These are your x1 values. I could simply change the scalar value to obtain a different scenario, say x2 values. I wasn't sure how to save the different scenario values, x2, as they would overwrite the previous values in the x1 scenario values. Hope that makes sense.