Adding custom fields to bulk send?

Following the guide found here - https://helpx.adobe.com/sign/using/mega-sign.html

The video never explains how to add the custom fields or extra fields that are added to the csv besides email, name, etc.

In the video above, if I wanted to add the field Phone, how would I add that object to the PDF when adding the custom name and signature fields?  When adding fields to the document would I pick a signer info field and change the name?  Text field?

My ultimate goal is to fill the csv with a unique serial number for each recipient and add that to the PDF requiring signature.  I've named the field in the csv as  _es_signer_serial and tried many different fields and changing their name to serial but it's blank when sending.