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

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

 

 

 

 The new project already references the essentially needed 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.