Hi Edwin,
Thank you for reaching out. You could use the KeywordPlan resource to get all the details related to Keyword Plan including the keyword plan id parameter. You could then use this keyword plan id to retrieve the historical statistics data. For your other concern, the client customer id is the same as the one you are referring to which is in the format XXX-XXX-XXXX. Let us know if you need additional information.
Regards,
Nikisha Patel, Google Ads API Team