A lawn care products company is rebranding their main product line, and they want to promote their video ads through home care inventory. Before they start setting up their campaign in Campaign Manager 360, what's the initial step they should begin with?

The initial and most crucial step before setting up a campaign in Campaign Manager 360, especially when dealing with video ads and specific inventory like "home care," is to confirm the requirements from publishers for specific campaign dimensions and other specifications. Publishers often have unique ad unit sizes, video formats, and technical guidelines that must be met for ads to serve correctly. Failing to do this upfront can lead to creative rejections, delays, and wasted effort. Creating HTML5 companion banner ads (A) is a subsequent step after understanding publisher requirements. Sending video ad tags to publishers (B) is also a later step, as tags are generated after the campaign and creatives are set up in Campaign Manager 360. Adjusting and uploading video ad dimensions (C) is part of the creative setup process, but it must be informed by the publisher's specifications first.