eighth.js

(function() {
  var form = document.querySelector('form[name=activity_select_form]')
  var matches = document.location.href.match(/\/bids\/([0-9]*)/)
  if (form && matches[1]) {
    form.innerHTML += '<input type="hidden" name="bid" value="' + matches[1] + '" />'
    form.action = 'https://iodine.tjhsst.edu/api/eighth/signup_activity'
    form.target = '_blank'
    form.addEventListener('submit', setTimeout.bind(null, window.history.back.bind(window.history), 1000))
    document.querySelector('div.blurb').innerHTML += ' API Signup enabled.'
  }
})()

manifest.json

{
  "manifest_version": 2,
  "name": "Iodine Eighth Signup Tool",
  "content_scripts": [
    {
      "matches": ["https://iodine.tjhsst.edu/eighth/*/choose/*"],
      "js": ["eighth.js"],
      "run_at": "document_end"
    }
  ],
  "version": "0.0.6"
}