Hello,
This feature was created specially for our demos using our own plugin for that. But if you need to make the same button you should make some customization on your site. We would recommend you to install -> wordpress.org/plugins/insert-headers-and-footers/ and add custom html of button inside one of area and then using custom styles (from child-theme/style.css / from theme options -> Theme styles / from the same section inside custom scripts ) add your styles for that specific button.
Examples:
1/ Backend -> https://prnt.sc/1ia81mh
Custom button text
.my-custom-sticky-button {
position: fixed;
bottom: 30px;
right: 90px;
z-index: 9999;
}
2/ Frontend -> https://gyazo.com/af483d0d085f2651753fc5468bd81a18
Regards