Navigate to your DFP your Google Ad Manager AdServer Login, edit the page layout and display the related list "AdServer Custom fields" and save the layout.
- At the related list "AdServer Custom fields" click on the button "New".
Enter the following information for the new record:
a. Select the Custom Field Name as desired.
b. The Custom Field ID can be grabbed from the URL (see next screenshot).
c. Enter in Data Field Path the API field name as described in the fields Helptext, e.g. ADvendio__Campaign_Item__c.YourCustomField__c
Enter the field path for getting the data being set to the custom field in the Ad server. The path must start with an object API name. Currently only "Media Campaign" (ADvendio__MediaCampaign__c) or "Campaign Item" (ADvendio__Campaign_Item__c) are supported as objects.
How to find the custom field ID in DFPin Google Ad Manager
a. Click on Admin and Global Settings.
b. Click on the Custom fields Tab and then create a new custom field or click on the name of an existing custom field.
c. Copy/grab the ID of the custom field out of the URL.
Until version 2.108: only location = Order and location = Line Item are supported.
Version 2.109 and up: location = Proposal and location = Proposal Line Item are also supported.
To map the standard field PO Number please take a look here: 184.108.40.206 Transfer Offer number and Advertiser External ID to DFPGoogle Ad Manager