Press on the "Connect" button in the API panel located in the Settings > Integration page to activate & enable the configuration of the custom fields that you are pushing through the API for a customer or invoice record:

Go to Settings > ERP Custom Fields page to configure the customer level or invoice level custom fields. 

If you are pushing the fields "customer_group" and "customer_industry" as part of the customer record, your customer level custom field configuration would look like this:

For the API technical development, refer to our Swagger UI to review the API definition for customers and invoices:

• Refer to: /post/customers (click on Upload section)
• Refer to: /post/invoices (click on Upload section)