Spaces are not for posting ads of any sort.
Spaces are simply a place to talk about whatever that space is dedicated to, localized discussions that can be followed or unfollowed to show on your feed or not.
If you want to place ads, use the hashtags #ads etc. Click channels at the top of every page to see some popular ones. Just write an update right in your own profile, and the use of hashtags will distribute it across the site to everyone following that hashtag.