JSFiddle - React, Tailwind, and code Playground

HTML

<form>
  <input type="checkbox" id="product1" name="product1" value="12">
  <input type="checkbox" id="product1" name="product1" value="13">
  <input type="checkbox" id="product1" name="product1" value="14">
  <button type="submit">Subscribe</button>
</form>

JavaScript

const form = document.querySelector('form');

form.addEventListener('submit', e => {
  e.preventDefault();

  const values = Array.from(document.querySelectorAll('input[type=checkbox]:checked'))
    .map(item => item.value)
    .join(',');

  console.log(`test.com/addtocart?${values}`);
});