How can I add custom fields to the composer?

[Don]

You can do this in category setting.
/c/category-slug/edit/topic-template

@renato

Applying a template to a category

Once you’ve created a form template, you’ll want to apply it to a category. To do this, head to the settings of the category you would like to apply the template to.

Select the Template menu, and then on the right, use the switch to turn on form templates. Once it has been enabled, you can use the dropdown to add one/more templates to a category.