Get preprocessor symbol UNITY_EDITOR for 3rd party Library

Actually there should never be a reason to use that preprocessor define at all. You generally should seperate runtime and editor code. Editor scripts (and editor assemblies) should be in an editor folder. Also keep in mind that the more recent Unity versions do have special import settings for assemblies which allow you to specify for which platform they are ment for / should be used for. The "editor" is also a platform.

It should be obvious that compiler pre-processor tags can't be used at all since they are only read by the pre-processor which pre-processes your code files before the code is even compiled. So the the actual compiler doesn't see pre processor tags at all. So of course you can't find any traces of those tags in a compiled assembly.

If for some reason you don't want to seperate editor and runtime code (though you should just like Unity itself: UnityEngine.dll and UnityEditor.dll) You would need to compile two versions of your assembly and put both in your project. The "runtime" assembly must be imported to not be used in the editor while the "editor" assembly must only be used on the editor platform. Though such a setup may work for you but i wouldn't recommend to use such an approach when you want to publish it on the asset store.

As i said it's generally a better approach to seperate runtime and editor code. Where / for what do you actually use the tags?