Forecasting with multiple Values

Hello!  I am trying to use the parameters and forecasting to get a revenue prediction for the future.... 

With that being said, I can never get the forecasting option to show up!  

My goals are (I just really need to complete one of them)

- is to have Jan2020 until March2021 revenue stay the same, and then the forecasted numbers are changing (April - December)

-  be able to use forecasting to see the next 8 months and everything changes.

Attached is a screenshot of what I have currently