title: Mimic Django's "add" button for related fields in admin
body: <p>Django has this admin feature for related fields when you press the "+" button, and you can add new data and then it attaches via jQuery, I'm assuming. I've looked at the code, and I could not find it anywhere.</p>
  
  <p>How would I mimic this in PHP?</p>
  
  <p><img src="https://i.stack.imgur.com/apMkk.png" alt="enter image description here"></p>
  
  <p>So, once you add the new 'Category', the ID attaches onto the 'Category' field in the window behind it. Then you submit the form and wala. How does this work? </p>