There are two ways to add an advertiser to your organisation.
Create a new advertiser
Go to Organisation Settings and select the Advertisers tab. Click Create New Advertiser and follow the steps. Once complete, the new advertiser will automatically appear under your organisation.
Add an existing advertiser
Go to Organisation Settings and select the Advertisers tab. Enter the Advertiser ID and click Add to confirm. To find your Advertiser ID, read How to find your Advertiser ID