You can start a discussion from two places. The box at the top of the main feed on the uni.xyz home page, Share something you're working on, opens a composer that posts under your name and puts your post at the top of the feed. The same box now sits at the top of every topic's own Discussions tab, and a post started there is automatically tagged into that topic, so it reaches people following it.
If you are an owner, admin or curator of an organization, the composer also shows a Post as picker above the post button. Leave it on "Post as yourself" to post under your own name, or choose the organization to post under its name and avatar instead. Everyone else does not see this picker and always posts as themselves.
What the tab shows
A topic's Discussions tab is the main feed narrowed to that one topic: posts, and activity on projects, journals and publications carrying it. Posting from either place lands in both views.