Need help adding Custom-CSS to a Form code block

Hey! 

I am trying to figure out how to recreate a particular form style. If you go to https://www.gatherandforge.com/get-started and click the get started button in the middle of the page, you will see the form. The form has three dots on the side, and you can also "scroll" through it. I am using a Macbook, so I can swipe up and down. I believe these forms are created using HubSpot, and I know how to create a form in HubSpot and in Squarespace. I just do not know how to do the coding to make that effect.

Thank you for your time and help in advance! 

I tried clicking 2 buttons on site but can't see 3 dots.

Can you take a screenshot of button?