The Squarespace Website Footer only allows you to add either plain text content or valid HTML/XML code within that area. Therefore, in order to add links within the footer you need to add the HTML code for this.
There are 2 different ways you could go about this. No matter which way you decide to add links to your footer, you'll need to remember to enter Structure Editing Mode (click the cubes icon in the upper right corner of your screen), and click the Edit Website Footer link to open the footer for editing.
1. Create Your Own HTML
If you have a little coding knowledge, or if you study some basic HTML tutorials (like the tutorial from a site like W3C) you can create your own code and add links to the site footer.
Something basic like this will work to create a link in HTML code:
<a href="page-in-your-site">This is the link text.</a>
Or to link to an external site:
<a href="http://www.externalsite.com">This is the link text.</a>
This will create a basic link. More complex link structures can be created with advanced HTML as well.
2. Copy Code for WYSIWYG Content
For those that don't have HTML experience, there is an alternate way to get the proper code into the website footer to create your links. This is done by adding links to a page via the WYSIWYG editor and copying the code that's generated, to be pasted into the Website Footer.
You could add a blank HTML page to your site and then navigate to that page. Enter Content Editing Mode (click the A icon in the upper right corner of your screen) and click the Edit Page link to open the WYSIWYG editor.
Once the editor is open, you can create links in the page with the editor tools. Once you add links in the WYSIWYG, it should look something like this:
Next, you'll click the small HTML link in the editor toolbar to view the HTML source code for those links you've just added. It will then look something like this:
Once you've accessed the HTML code, you'll want to copy that code (highlight and CTRL+C on Windows, Command+C on a Mac) so that you can then paste that into your footer.
You can close the editor or cancel out without saving (since you just created the page to use for making your links).
To add this code to your footer, enter Structure Editing Mode (click the cubes icon in the upper right corner of your screen) and click the Edit Website Footer link to open the editor for the site footer.
Paste your code there (CTRL+V on PC, Command+V on Mac). Once you save the changes to the footer, you should see your links displaying just as you created them in the WYSIWYG editor.