If you're interested in creating a post that will be displayed with the mention 'Sponsored by' added to it, here's how you need to proceed:
Open up a new Campaign Creation Flow, and get to the creatives step.
You should already have either chosen or created a new creative for your campaign, and you'll notice there is, lower in the page, an advanced section. Open it, and there will be an option to select from a Page that will be Sponsoring your post! Simply type the name of the page, and you will be able to select it!
Select the Page you want to use, and simply move on with the flow! As you select Pages, the Live preview will update to show you what it's looking like.
The end result will look like this: