Indent paragraph text?

You can use my theme-component as an example:

https://github.com/MonDiscourse/md-composer-extras

You have two options, add an icon to the toolbar or on the gear menu

Toolbar:

api.onToolbarCreate(toolbar => {
    toolbar.addButton({
        id: "align_justify_button",
        group: "extras",
        icon: "align-justify",
        perform: e => e.applySurround('[wrap="justify"]\n', '\n[/wrap]', 'align_justify_text')
    });
});

Gear menu

api.addToolbarPopupMenuOptionsCallback(() => {
  return {
      action: "floatleft",
      icon: "indent",
      label: "float_left_button"
  };
});

You can have some extra options with a plugin, but the component should be enough : GitHub - MonDiscourse/discourse-formatting-toolbar: Add formatting options on your post (center, align to right, justify a text, etc.) · GitHub