Adding Calendly pop-up under header button

Hi Squarespace family,

I'm interested in adding a Calendly pop-up when clicking the 'Book a Call' button, in my website header. As standard options I can only link it to another page in my website or to an external URL. Any idea on how to implement this custom behavior? Any help would be appreciated. 

The code I'd need to insert is:

<link href="https://assets.calendly.com/assets/external/widget.css" rel="stylesheet">
<script src="https://assets.calendly.com/assets/external/widget.js" type="text/javascript"></script>
              <a href="" onclick="Calendly.initPopupWidget({url: 'https://calendly.com/bookacall/30min'});return false;">
                Book a Call
              </a>

Thanks a lot!

PS: This is how the website menu looks like