How to create a plugin with backend API calls to populate composer while drafting?

[Arkshine]

@fabkosta

Question: Where can I find docs for DModal, DButton, and other such Discourse-specific objects?

Here:

Using the DModal API to render Modal windows (aka popups/dialogs) in Discourse

Discourse 3.1.0.beta6 ships with a brand new <DModal> component-based API. This supersedes the old controller-based API, which is now deprecated. If you have existing modals using the old APIs, check out the migration guide here. Rendering a Modal Modals are rendered by including the <DModal> component in a handlebars template. If you don’t already have a suitable template, check out Using Plugin Outlet Connectors from a Theme or Plugin. A simple modal would look someth…

Also worth noting:

Discourse toolkit to render forms

Basic Usage FormKit exposes a single component as its public API: <Form />. All other elements are yielded as contextual components, modifiers, or plain data. Every form is composed of one or multiple fields, representing the value, validation, and metadata of a control. Each field encapsulates a control, which is the form element the user interacts with to enter data, such as an input or select. Other utilities, like submit or alert, are also provided. Here is the most basic example of a fo…

You can look at Discourse/plugins/theme components for examples.