csml | Like C STL but with macros and C | 3D Printing library

You need to iterate through the Array in order to have a component-compatible list of Card() components, here is the doc for reference: https://docs.csml.dev/studio/channels/slack/message-formats#carousel-card

So the code below would work:

I have never used VSTO myself, but from the documentation page architecture of VSTO add-ins it looks like the add-ins are normal .NET managed assemblies packaged with some additional metadata. This suggests that you could have a main assembly in a .NET language such as C#, developed using the Visual Studio tools, that interacts with an OCaml codebase using CSML as a bridge.

You could try doing this by creating a simple "hello world" VSTO add-in, that internally invokes a simple "hello world" example of CSML usage to call a very simple OCaml library. If this doesn't work, I would suggest asking the CSML people directly for advice; if it does work, you could send them the example for them to describe/integrate in their documentation.