Only admins can do that at this point (Discourse restriction).
To make any post into a wiki (which then all users can edit), do the following after it has been posted:
- Click the ellipsis (
...) on the post to expand the full menu:
- Select the Admin-Options by clicking the wrench:
- Select "Make Wiki" in the menu:
This will make the post into a wiki post, as indicated on the top right from now on: