Question How to use a .dll plugin in C# script

So I have a plugin in the .dll form in the plugin folder but I have no idea on how to call its methods from visual studio and get any autocompletion whatsoever.

 

If it's a managed (C#) DLL, then it should just work. If it's a native plugin (like C++) then you have to declare extern method signatures with the DLLImport attribute. https://docs.unity3d.com/Manual/NativePlugins.html

 

What plugin is this?

A random DLL won't just magically work within Unity.

A software that provides (some of) their functionality as .NET DLL should provide you with documentation on how to call its functions from C#. If there is no such documentation or the examples aren't for C# you can almost bet it won't work with Unity.

Even if the software specifies that it supports Unity it may not support all editor versions or target platforms.