Actually, Prompt dialog page can be used only with Extensible property set to false.

In our scenario, a customer would like to add their own prompt-guide, let's say frequently used (predefined) prompts, so that users knows what they can ask to Copilot and in which way.

So in order to add custom actions to the prompt-guide we need the prompt dialog page to be extensible.

This is a common scenario useful also when using you Sales Lines Suggestion capabilities.

Link to Yammer discussion: https://www.yammer.com/dynamicsnavdev/#/threads/show?threadId=3181468706807808