Add auto zip code vlue in megento checkout page using javascript

Hi here is sample page. Which have a required field Zip code, in this field, I want value automatically populated using javascript.

sample product page > add to cart > Proceed to Checkout || then you get the checkout page.

I try this methood but dosen’t work
document.querySelector('div#shipping-new-address-form div:nth-child(8) input').value = '12345';

Please suggest me how can I solve this issue.