Does anyone know how to populate the Vendor Name "Entel" to second-line under the GL impact tab? Is this require any custom script? Or is there any config setting we can check?

Your question is vague. You can populate a select field in the UI, via workflow, script, etc. This is likely a List/Record field of type Vendor. Although I wouldn't know, haven't touched the GL.

This is the field that books expenses attributable to Customers...as designed this would make the line item available to bill to the customer on their next invoice run.

What is the use case by the way? Maybe a saved search will do, etc.

@Luis the use case is we sync expense report from concur to netsuite to create a credit card charge via REST. this GL impact is auto populated after we fill in the vendor, account, transaction amount, cost center and subverticals. Wondering how do we populate the "Vendor" in the second line under this GL impact