Enabling credit at the advertiser level can be done within the advertiser's window.
From the Main Menu, Go to Advertisers -> Advertisers. Click on the Advertiser's name to open the Advertiser Window.
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From the Advertiser's window toolbar. Click on Enable Credit...
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You will need to confirm Enabling Credit at the Advertiser Level.
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When you confirm enabling credit at the advertiser level. The advertiser's credit will be shown right next to the advertiser's name.
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Note:
Enabling Credit at the advertiser level won't affect any contracts. You will still need to enable credit at the contract level to use the advertiser credit.