phpBB 3.1 extension that allows you to set a "posting template" on per-forum basis. The posting template is a predefined "text", that will appear if a user starts a new topic in that forum.
This extension is the 3.1.x version of the 3.0.x posting template
Clone (or download and move) the repository into the folder ext/phpbbmodders/postingtemplate:
cd phpBB3
git clone https://github.com/phpbbmodders/phpbb-3.1-ext-postingtemplate.git ext/phpbbmodders/postingtemplate/
Go to admin panel -> tab customise -> Manage extensions -> enable Posting Template
Visit each forum and in the general settings you can set a posting template for that forum.