Once you create you project and are in edit mode, drop a container onto the top of the page.  Select the 'Style' tab in the top right and scroll down to the bottom to find the 'Advanced' section.  You will be adding to CSS values here.

Next, add another container that will scroll with your content.  Add the following CSS value to this container as well.

Fun Fact: If you are using a phone or tablet page type, you may need to wrap all your page content in one container.  Like so:

If you have any questions, feel free to contact Support.  

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How to Create a Fixed Footer