As the others have indicated, it's at Project | Show Project Targets.
Screen Shot 2018-05-17 at 1.08.08 PM.png (11.72 KiB) Viewed 1503 times
If you don't see the little blue Twitter icon, you'll need to go into Options to turn it on.
Screen Shot 2018-05-17 at 1.08.16 PM.png (35.67 KiB) Viewed 1503 times
Edit: Hmm, I notice from your screenshot in the other thread that you don't seem to have the Twitter option in the target options. Not sure what would cause that. You might need to set up your Twitter account in your mac System Preferences | Internet Accounts
I recently learned how to go into system preferences and turn on posting from Scrivener to Twitter, but yesterday I downloaded Mojave and now I am unable to post from one to the other. Worse yet, when I go into system preferences to add Twitter again, it will not allow me to. Will Mojave not allow this action?