Topic templates are a useful tool to reinforce forum conventions or to guide new users.
Category-specific topic template
Some categories might follow a strict pattern. One such example would be a Movies category, wherein you’d like users to start new topics according to a certain template:
I saw: insert movie title
I think it was worth: how many stars out of 4 stars
Review here:
To apply this template to all new topics posted in this category, go to a category, click “Edit” and find “Topic Template” in the top menu.
You can see a live example of this if you register on our Try sandbox forum and post a new topic to the Movies category.
Pre-fill topic by URL
It’s also possible to create URL that, when clicked, will open a pre-filled topic. To compose a new pre-filled topic, modify URL params like this:
http://discourse.example.com/new-topic?title=topic%20title&body=topic%20body&category=category/subcategory
This will open composer window pre filled with topic title, body and desired category.
This can also be tested live on try.discourse.org:
https://try.discourse.org/new-topic?title=topic%20title&body=topic%20body&category=discourse
Also, instead of providing category/subcategory name, you can also specify category_id, like:
https://try.discourse.org/new-topic?title=topic%20title&body=topic%20body&category_id=11
Note that if both category_id and category is passed as param then priority is given to category_id.
Source: https://meta.discourse.org/t/what-are-topic-templates/38295