TopicDefaultTag is a plugin to have tags (even staff-only tags) automatically added to topics in a category.
Follow Install a Plugin
how-to from the official Discourse Meta, using git clone https://github.com/pfaffman/discourse-topic-default-tag.git
as the plugin command.
To use, add a default tag in the category settings.
If you have issues or suggestions for the plugin, please bring them up on support.literatecomputing.com.