The post button appears in forum categories automatically. You can customize the forum post button and other forum buttons on the Configure Site> Social Networking > Forum page.
How to add Forum Categories:
An easy way to do this is to run the Templates generator for Forum Sample Data. Then you have a sample category - you can edit this from Pages > List Pages, by clicking on the Edit button for that category page. And you can add new forum categories by doing Add Page, and on the top right there is an option "Populate Fields With Page:" choose the forum category you already have, to copy it, and then update it as needed.
Alternatively, you can add it from scratch, as outlined on the user manual topic, "Create, Customize, & Integrate Message Boards (Forum) For Your Website" - https://www.ultimatewb.com/message-boards-forum-app/create-customize-integrate-message-boards-forum