How do I add a button next to the reply_to_topic button

[Moin]

There is a guide about using plugin outlets

Using Plugin Outlet Connectors from a Theme or Plugin

Discourse includes hundreds of Plugin Outlets which can be used to inject new content or replace existing contend in the Discourse UI. ‘Outlet arguments’ are made available so that content can be customized based on the context. Choosing an outlet To find the name of a plugin outlet, search Discourse core for “<PluginOutlet”, or use the plugin outlet locations theme component. (e.g. topic-above-posts). Wrapper outlets Some outlets in core look like <PluginOutlet @name="foo" />. These allow …

For more information about developing theme components take a look at Theme Developer Quick Reference Guide