Add short code to HTML page?

Hi, all:

I made a custom landing page in HTML/CSS. Here’s a link:

http://machinesandwords.com/welcome-agency/

I know the code is terrible. I don’t know what I’m doing, and I don’t know PHP, so please stop laughing and pointing.

I tried making the CSS into a child theme, but couldn’t get it to work without overriding the parent theme. I got scared and gave up on that and just made a folder for the HTML/CSS pages in my root dir. So this page is outside of the WP directories and is not populated by the parent theme.

I want the user to be able to click the big ‘PICK A TIME’ button, which would then open a lightbox presenting a calendar. I’ve installed Appointments Lite by wpmudev and the calendar works fine on a regular WP page.

I’ll worry about the overlay/lightbox next, but I want to be sure it’s possible to make the calendar open from the HTML/CSS page before I spend time figuring that out.

So is it possible to link my big PICK A TIME button to the plugin via short code in HTML?

Does the HTML/CSS file have to be in the WP-CONTENT dir for that to work, if it will work at all?

Thanks!
Jay