How to Set External ID via Script?

Hi There

I am trying to set the external ID value on a customer record using a userevent script.

I have tried with different entry points--before submit and after record submit.

There is no error message though the external id is not being updated either.

this is the script:

/**
 *@NApiVersion 2.x
 *@NScriptType UserEventScript
 */
define(["N/record"], function (record) {
    /**
     * @param {UserEventContext.beforeSubmit} context
     */
    function beforeSubmit(context) {
        var customer = context.newRecord
        var customerId = customer.getValue({ fieldId: "entityid" })
        var id = customer.id
        log.debug("id", id)
        
    
        var externalId=customer.setValue("externalid", customerId)
        // customerNew.save()
        log.debug("customerNew", externalId)
    }

    /**
     * @param {UserEventContext.afterSubmit} context
     */
    function afterSubmit(context) {
        var customer = context.newRecord
        var customerId = customer.getValue({ fieldId: