How to create a sticky button that stays on the bottom of the page?

Hi, I'm looking to add a button that always shows on the bottom of the screen that says "Subscribe to our newsletter", and when clicked it kind of pops up some text fields where users can input their name & email to sign up. Haven't published my site yet. Not sure if Squarespace has this feature or if it has to be custom HTML/CSS.

Example: basically exactly like on this site - https://www.betterdaysrescuefund.org/ 

Hey ctaylorr!

You can actually do this using Squarespace tools, however, initially, it's not a button in the bottom as you might have to write additional coding for that, however it does have the rest of the functionality:

https://support.squarespace.com/hc/en-us/articles/115008375848-Creating-a-promotional-pop-up

1. Click on Marketing

2. Click on Promotional Popup

3. Click on Change Layout

There are a few codepen solutions you might could implement:

https://codepen.io/syalakshay/pen/pWmYXx

https://codepen.io/corinneling/pen/mLwKaG

Awesome, thank you so much!