var record = nlapiCreateRecord('customrecordtype');
record.setFieldValue('recordname','New Custom Record');
record.setFieldValue('scriptid','_my_custom_record');
var id = nlapiSubmitRecord(record);

var field = nlapiCreateRecord('customrecordcustomfield',{'rectype': id});
field.setFieldValue('fieldtype','TEXT');
field.setFieldValue('label','Custom Field');
field.setFieldValue('internalid','_my_custom_field');
nlapiSubmitRecord(field);