Compared to using a text editor and the compiler provided by the .NET Framework, it is much easier to create an add-in with using a development environment such as Visual Studio 2022.

Eplan templates are installed in Visual Studio with the API setup, which can be downloaded from the EPLAN homepage. 
To create an add-in, just create a project in Visual Studio using the "Eplan Api Addin" template from C# Projects: 

 

 

 

 The new project already references the essential EPLAN API assemblies and a file with the module class: 

 

 

 

You can add a new Action class by the Add New Item menu point and selecting the template "Eplan Action": 

 

 

For Visual Basic, the work flow is identical.