Add Template Listeners
From phpBB Development Wiki
What are template listeners?
How to use one
I'm assuming you already know what the events's identifer is. If you don't then you should look it up on the Events_List. If it isn't on the events list you can add it as described in the Adding_Events article.
So lets say our event is called overall_header_head and injects into overall_header.html in the head element.You create a file in your extension in
You essentially call the file by the event identifier followed by _listener.html and place it in the event/ subdirectory of the style's template folder.
Then you type in the content you want to be injected where that event is and you're good to do. It's as simple as that.
If you want it to work for all styles (recommended) then instead of style name use `all`.