N/currentRecord Module
Field.insertSelectOption()
Method Description: Inserts an option into certain types of select and multiselect fields. This method is usable only in select and multiselect fields that were added by a front-end Suitelet or beforeLoad user event script. The IDs for these fields always have a prefix of custpage.
Returns: Void
Governance: None
Supported Script Types: Client scripts For more information, see SuiteScript 2.0 Client Script Type.
Module: N/currentRecord Module
Since: 2016.2
Search NetSuite - https://system.netsuite.com/app/help/helpcenter.nl?search=Field.insertSelectOption(options)

// Code Example 1

//Add additional code

//...



// Instantiate the field. Note that this method is supported only

// on fields whose fieldIds have a prefix of custpage.



var field = call.getField({

    fieldId: 'custpage_select1field'

});





// Insert a new option.



field.insertSelectOption({

    value: 'Option1',

    text: 'alpha'

});

//...

 //Add additional code



//SOURCE: https://system.netsuite.com/app/help/helpcenter.nl?fid=section_4779675098.html