JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form">
    <input type="hidden" id="delterms" value="" name="O_" />
    <input type="checkbox" id="checkbox" /> Accept?
</form>

JavaScript

var checkbox = document.getElementById('checkbox')
  , hidden = document.getElementById('delterms');
checkbox.addEventListener('change', function() {
  hidden.value = (this.checked) ? 'Accepted' : '';
  alert("Hidden value: '" + hidden.value + "'");
}, false);
console.log(hidden.value)