How do I define a custom modeling toolbox item?

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C#

Visual-Studio

I want to define a custom modeling toolbox item but I can't find a way to produce the following files: .classdiagram and classdiagram.layout.

What I have tried:

i tried to reproduce the steps described in the foolowing msdn article:

Define a custom modeling toolbox item - Visual Studio 2015 | Microsoft Docs[^]

I cannot go ahead and perform the following activity: copy both diagram files, one with a name that ends "…diagram" and the other with a name that ends "…diagram.layout".