In which plugin outlet do you want to put your banner?
Take a look at Using Plugin Outlet Connectors from a Theme or Plugin
You could use Add a featured topic list to your Discourse homepage as an example
You can use the Plugin outlet locations theme component to find the names and locations of the plugin outlets. “above-main-container” for example is the top one above the topic list, but not above the sidebar.
Looking at the code of a Theme component which adds something similar is also a great way to learn.