FString path = FPaths::ProjectDir();		//get relative path for project
FString fullPath  = FPaths::ConvertRelativePathToFull();	//to full path

FString exePath = "Co/hello.exe";
FString finalPath = fullPath + TEXT("/") + exePath;		//hard

FString finalPath = fullPath / exePath;	//easy
//or
fullPath.PathAppend(*exePath, exePath.Len());

FString path = TEXT("F:\\PJ\\hh\\Saved/d/c.exe");		//not stardard
FPaths::NormalizeFilename(path);