How to load a custom record which has parent as customer record

I created a custom record which has parent record as Customer.

This custom record will have many lines, each line will store values of few fields.

I'd like to write a script to add new line to the custom record of a specific customer and set values to the fields of the new line.

Here is my custom record:

And here is my script:

var customerRecord = record.load({

    "type": record.Type.CUSTOMER,

    "id": customerId,

    "isDynamic": true

});

customerRecord.selectNewLine({

     sublistId: 'customrecord373'

});

customerRecord.setCurrentSublistValue({

     sublistId: 'customrecord373',

     fieldId: 'custrecord27',

     value: 'test' 

});

customerRecord.commitLine({

     sublistId: 'customrecord373'

});   

var recId = customerRecord.save();