Linker error - Not finding implementation

#include "IPluginManager.h"

error LNK2019: unresolved external symbol "__declspec(dllimport) public: static class IPluginManager & __cdecl     IPluginManager::Get(void)" (__imp_?Get@IPluginManager@@SAAEAV1@XZ) unresolved to which reference is made in the function "private: static class FString __cdecl  FXXXXXToolStyle::InContent(class FString const &,char const *)" (?InContent@FXXXXToolStyle@@CA?AVFString@@AEBV2@PEBD@Z)

In order to access the IPluginManager.h header file you need to add the “Projects” dependency on your plugin’s dependencies That means opening the xxxx.build.cs and adding to Private dependencies “Projects” on the list

PrivateDependencyModuleNames.AddRange(
	new string[]
	{
		"CoreUObject",
		"Engine",
		"Slate",
		"SlateCore",
        "Project",
		// ... add private dependencies that you statically link with here ...	
	}
);