I’m a total newbie, but this worked for me, sort of. It allows you to view the first post, provided you clicked on a menu item to get there, like if you were a visitor to the site. If you try to see it from the admin dashboard Forums, the first post won’t show up.
Make a new topic. Give it the same name you will give to the Forum itself. Make it sticky. Publish.
Make a new forum. In the text field, click the link button. A list should appear. Link to the topic called whatever you called it. On the next line, write your first post. Copy that post text to use later. Publish.
Make a new page called Forum, and put that new page in a menu.
Edit the page called Forum (we’re not editing the forum itself, just the page called Forum): In the text field, click the link button. Link to the topic. Publish. Then paste the text that you copied from the forum. Publish.
View the page called Forum. Then view the Forum directly from the dashboard.
If that doesn’t work, try Settings > Permalinks > Save Changes.
Please let me know if any of these steps is unnecessary.