updateAmazonTags.js

// grab all links on page and convert Nodelist to array using spread operator
const links = [...document.querySelectorAll('a')]
// filter links that contain specific word
links
    .filter((i) => i.href.includes('amazon.com'))
    .forEach((i) => {
        // convert string href to URL
        let link = new URL(i.href);
        
        // update params if present or add new params if not present
        link.searchParams.set('tag', 'test-tag');

        // update the url on page
        i.href = link.href;

        // log to see update links
        // console.log(i.href);
    });

/*  you can remove the .filter part if you are selecting links on page like
    document.querySelectorAll('a[href^="http://www.amazon.com"], a[href^="https://www.amazon.com"], a[href^="http://amazon.com"], a[href^="https://amazon.com"]');
 */