I have the following function that runs when the page is loaded, but I want it to only run when a button is clicked. How would I go about doing that?

Code:
<script>
  $(document).ready(function () {
          $.fancybox({
              'width':          '80%',
              'height':         '80%',
              'autoScale':      false,
              'transitionIn':   'fade',
              'transitionOut':  'fade',
              'type':           'iframe',
              'scrolling' :     'no',
              'href':           'cmetogo-offer.html'
          });
  });      
  </script>
And this is the anchor tag that I would want to trigger it:

HTML Code:
<a href="index.html">Login Here</a>
Any suggestions?