When Campaign Manager isn’t used as the ad server, what should Search Ads 360 advertisers link to in Campaign Manager?
If the advertiser doesn’t exist in your Campaign Manager account, you will need to create the advertiser first. Note that each Campaign Manager advertiser in your Campaign Manager account can be linked to only one Search Ads 360 advertiser. Learn more about the one-to-one relationship between Search Ads 360 and Campaign Manager.
Read more here: https://support.google.com/searchads/answer/2491917