If you’re at least trust level 1 you can send an email to category@discourse.gnome.org with [tag] in the subject, for the category and tag you want to create a topic for. That may not be enabled for all categories.
To reach a higher trust level you have to use the website. The trust levels are explained here. Basically: you need to enter 5 topics, read 30 posts and spend a total of 10 minutes reading posts to reach trust level 1.
You can use Discourse entirely through email, but that’s not the main way to use it. Emmanuele’s reply here gives more context: Sub Categories? - #6 by ebassi