I'm sure this is easy, but I don't know SS very well. I repurposed another working script to try to do the same thing (Set Ext ID equal to a custom field value on the record) on a different record type and it's not working. When I save the record in the UI, I get "Record has been changed" error and the value does not get set to Ext ID. The Ext ID has been set, but it's a bunch of gibberish vs the data I'm trying to set it to. On the SS Execution Log, it says just "Set Ext ID of standard record.", which I assume is coming from the line in the code, but it shows as Type=Audit and not error.

define(["N/record", "N/log"],

function(record, log) {
    function setItemExternalID(context){
      var itemRecord = context.newRecord;

      try{
        var itemField = itemRecord.getValue({ fieldId: 'cust_field_reference_i_triple_checked_is_correct'});
        
        if(itemField){
            record.submitFields({
                type: record.Type.NON_INVENTORY_ITEM,
                id: itemRecord.id,
                values: {
                    externalid: itemRecord
                }
            });
            log.audit({title: 'Set External ID of ' + itemRecord});
        }
      }
      catch(e){
        log.error(e);
        log.error(e.message);
        log.error(e.stack);
      }
    }

    function beforeSubmit(context) {
      setItemExternalID(context);
    }
    return {
        beforeSubmit: beforeSubmit
    };
});

you are setting the

externalid

with the value of

itemRecord

and not

itemField

before submit occurs before the record has been saved

You should consider keeping the script running before submit and using itemRecord.setValue(). This is significantly more performant than record.submitFields(). (Do not add itemRecord.save() though as this is automatically run by default after before submit scripts run.)