Simple how to conditionally display field

Hi- I'm brand new to SuiteScript. What I've been asked to do is really simple, but I'm not sure if it can be done in our Netsuite account.

We created a custom checkbox field for timesheets. When the customer field loses focus I want to grab the job id (or even the text would work) and if it's a particular number then I want to make visible the checkbox column. We want to have the checkbox only apply on a couple of customers/jobs.

Normally this is very trivial, I'd do something like :

document.addEventListener('DomContentLoaded', function(){
    var jobElement = document.getElementById('someId');