@tuanphan @WHOisDANE Were you ever able to find a solution? I was trying to achieve the same thing, to add a Calendly popup to my header actions button.
I attempted a workaround by absolutely positioning a code block that I added to the footer in the top right of my screen (so it looks like the header button, but it's actually not). However, the button is not working on the homepage or the "what we do page" - but it is working on all other subpages. Sometimes clicking on the button prevents scrolling on the page altogether, so I have to find another way.
Has anyone found a way to use the standard header button but call a Calendly popup?
For reference, my site is: https://iced-media-redesign.squarespace.com/ (password: icedmedia)