Custom button

Hi, I wonder how I can add a custom button via shortcode? Example, this site https://tinyurl.com/2tpsp7uv has added a custom button which will call the ‘Contact’ circle which is a similar plugin as yours. I wonder if you can provide this custom link code for me?

Thank you.

The page I need help with: [log in to see the link]