The Fallback URL is used for unsold, cookieless traffic that has been returned from the ad network.
You need to set up the Fallback URL after checking if there is any unsold traffic in the ‘Back & Fallback URL’ report here. If you do not see any activity there, then you do not need to set up a fallback.
To add Fallback URL you need to press "Add creatives" right next to the Fallback campaign:
We recommend using creatives of Ad Networks with 100% fill rate or your own websites.
For Ad Networks creatives, choose an Ad Network name from a dropdown list, click the ‘link’ button, and select creative to link. For your own website add ‘Link URL manually.
NB! For fallback use creatives and websites which have not been used elsewhere before.
You may add more than one creative.