How do you add a third-party lib to the engine

So I do not want to add a third-party lib to my unreal project. I’m trying to add a third-party lib to the engine. (I’m building the engine from source.) I’ve already got it working by modifying a .build.cs file, because I’ve added the include paths, the additional library, the public library path, and the run-time dependency. What am I missing? When the editor comes up, it can’t find the DLL unless I add its directory to the PATH env-var. Also, I can build a package for my game and my DLL finds its way into the package’s third-party directory (along with lots of other third-party DLLs in their own respective folders), but the packaged executable can’t locate the new DLL either.

To re-cap, all I’ve done is move my third-party targets into the third-party directory and then point Unreal to those files by modifying a .build.cs file. My build is succeeding, but I fail at run-time. With my PATH env-var patched, I’ve been able to debug the engine and step into my third-party library code. It’s just that I’m missing some sort of config setting somewhere that sets up the search paths so that Unreal can find the DLL at run-time. Any clues?