In Mendix, an extension is more like a Studio Pro plugin than a widget. You typically build it using either the C# Extensibility API or the Web Extensibility API, depending on whether you want deeper IDE integration or a web-based UI.
The basic flow is simple: create a separate extension project, implement an extension point (like a menu or panel), build it, and copy the output into your app’s /extensions folder. After reopening Studio Pro, the extension becomes available (for example, in the menu or as a panel).
Project structure is not strict, but usually:
Unlike widgets, you don’t package it as .mpk. It runs directly inside Studio Pro.
Short summary: Create project → implement extension → build → copy to /extensions → reopen Studio Pro.
If this resolves your issue, you can mark it as accepted.