How to set field mapping for opportunity field in CRM quote?

We are integrating CPQ to Salesforce with reference to this page, Salesforce Integration Instructions - SAP Help Portal, with our own naming conventions, e.g. salesforce quote object (custom_quote__c), quote id (custom_quote_id__c), quote owner id (custom_quote_owner_id__c), opportunity id (custom_opportunity__c).

In the setup page, Setup > CRM Integration > CRM Quote, we can specify the object and field names for CRM Quote Object Name, Quote Id and Quote Owner ID. However, I couldn't find a place to specify the opportunity id. May I know how to specify the field mapping for opportunity in CPQ?

Thanks & regards,
Hulei