Navigate to 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
How to find the custom field ID in 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.