Forza Horizon 4

You will need to collect 2,600 bricks to unlock the Lego Speed Champions Senna event. Complete various races, speed traps, drift zones, and other Brick Challenges to reach this brick count. If you've made it this far into the game, it should come intuitively.

Once you reach 2,600, return to your Lego Master Builder's House to collect the Lego Senna. Then head to the blue race event icon to the left of the town center.

The race is pretty easy and should take you less than 2 minutes to complete, but Rewind is your friend if you need it.

The achievement should pop once you reach the Rewards screen that says LEGO® Speed Champions Senna event COMPLETE

its simple really story related as in just keep on collecting bricks and eventually there will be the senna speed champions event