First, you need to have unlocked these:
The first few hours of Forza Horizon 4
see you experience all 4 Seasons before you join the full "shared world".
As you complete races, challenges and other activities you earn Influence. This Influence is used to fill a progress bar which you see on your Map from the main menu, or as a pop-up while you're driving.
While in Horizon Spring you have 2 objectives for which you need to earn Influence:
> Qualify for the Spring Showcase event.
> Qualify for the Horizon Roster.
Fill up the Influence bar as you have done before to first unlock the Showcase event. With that done repeat the process to unlock this Achievement.