Assuming you enabled the plugins, you only need to add the modules to ProjectName.Build.cs. It should look something like this:
PublicDependencyModuleNames.AddRange(new string[]
{
"Core",
"CoreUObject",
"Engine",
"InputCore",
"OnlineSubsystem",
"OnlineSubsystemEOS",
"OnlineSubsystemEOSPlus",
"OnlineSubsystemSteam",
"OnlineSubsystemUtils"
});
The formatting does not matter whether it is a one line or multiple lines, but it should work when you include the headers without the entire path.