In the XOSync wizard there is not a place to connect Ontraport fields to tracking in Xero.

Learn more about tracking in Xero here:

When transactions from Ontraport are synced to invoices in Xero our script looks up the default tracking options on the Xero Contact and uses that information to update the tracking. 

If tracking is important to be added to all created invoices then we suggest that you add a default tracking code for contacts that are already in your Xero account.

For new contacts however we do not have a feature to automatically set the tracking on a new invoice for a new contact.

Tracking example: