In order to declare Content.ad an authorized seller of your inventory. Every publisher should add the following to the ads.txt file on the root of your domain.
You will need to replace DOMAIN_ID_GOES_HERE with the actual domain id for each site that you are running the Content.ad widget on. You can find your DOMAIN_ID by going to the Domains Section in your Content.ad account and selecting the ID to the left of the domain name.
content-ad.net, DOMAIN_ID_GOES_HERE, DIRECT