Cannot use function in dll

I’m a newbie. I use RuntimeDependencies.Add and PublicAdditionalLibraries.Add to add .dll and .lib to my unreal project. However, I don’t know what to use to load DLL function. So I use __declspec(dllimport) / __declspec(dllexport) to declare the function is in .dll and hope it work. But it doesn’t. So, how can I use the function in .dll? plz help me.