Please make sure to copy the value into the field Alternate Order ID of the media campaign. This will make sure that your offer number gets copied/transferred by the ADvendio Interface into the DFP standard field PO Number.
To map other custom fields, please refer to the instructions here: 7.3.4.1 Map and transfer data into DFP custom fields
To map the advertiser ID choose which field you like to transfer in the DFP Adserver login:
This will lead to filling out the following field in DFP