To Add From the Campaign Dashboard:

Navigate to "Campaigns"
Find the campaign you want to add a post to.
Select the "+ Add" button under Actions to add a post.
Copy and Paste the URL into the Prompt, then click Preview
Choose Submit.

To Add From Within the Campaign:
Navigate to "Campaigns"
Find the desired campaign and select View
Select the "Add Post" in the top menu bar
Copy and Paste the URL into the Prompt, then click Preview
Choose Submit.