This feature is to be found in Creative Manager.
Go to the campaign and open up the specific format you would like to preview
At the top, you have the "Save and Preview" button. Click on this button.
Now a new window opens up. This is how your banner looks when it's live.
Copy the URL and send it to your teammates, customers, etc. to get feedback.
OBS. The link automatically updates when changes are made in Creative Manager. Therefore, you only have to send this link one time.
When it's ready to go live, export the banner: