After you've saved an ad in Pro it's important to preview it on site to make sure everything looks good. Doing this is easy. Simply load up the ad detail page for the ad you'd like to preview (ad details pages include, "/ads/details/id/" in the URL). Once on the ad details page, in the top right hand corner, click the icon shaped like an "eye" (see screenshot below). Once clicked a flyout will appear where you can enter the URL where you'd like to preview the ad (see the notes below as to why this is important).
After you click the "Preview" button the URL you entered will open in a new window and you should see your ad, as shown in the example screenshot below.
A few notes: