I have read the following article. https://inspiringingenuity.net/2015/12/03/htmlsdk/. I have pulled apart several plugins in that are already installed on my machine. But I am about at the end of what I can self discover.
@amarrero45 I don't have any other formal documentation to share right now. Can you tell me a little more about what you are working on? Perhaps I can point you to some other resources that can help.
What it sounds like you're actually looking for is documentation for our Gallery API.
That's how we've gone about prototypes where we create an Excel plugin that runs and fetches data from Alteryx Analytic apps hosted on a Gallery.
Here's a blog post I wrote last year that contains detailed information on using the Gallery API with Python. While this isn't showing how to build an Excel plugin, it will go over what you need to know to build your own plugin in the language of your choice.
Hope this helps.
Started with the UI side but am blogging what I understand of the HTML/JS SDK.
Have the TypeScript type definitions for both engine and UI side written as well.