First, go to: https://app.datatrics.com/integrations/appnexus/ and press "Install"
You will see the following screen:
Fill in the username with admin rights, and the password. You will also have to fill in your unique Advertiser ID. The advertiser id is a unique ID that every advertiser has.
After filling in these fields and authenticating, we will be able to send unknown profiles from segments, campaigns and journeys.