Nah, I’ve just added it and now it says:
C:\Users\Alex\Documents\Unreal Projects\Game001\Intermediate\Source\Game001.Target.cs(2,1): error CS8805: Program using top-level statements must be an executable.
C:\Users\Alex\Documents\Unreal Projects\Game001\Intermediate\Source\Game001.Target.cs(2,1): error CS0103: The name 'BuildEnvironment' does not exist in the current context