The best way to pin a post is to move it to the Featured Section or Recommend it! As a Host or Moderator, when you click on the three-dot menu of posts on the web, you'll see an option to Recommend.
When you click Recommend, the chosen post will jump to the top of your members' Feeds. You will want to be sure that you have selected "Last Activity" as the default for the Feed feature in a Space.
You can Recommend posts in the Global Feed, as well as in Spaces.
Updated July 2022