To add a Secondary Navigation in Squarespace, you can follow these steps.
This will require a custom plugin.
#1. Install this plugin
Plugin will give you some code to add to Code Injection – Footer, like this.
#2. Use this code to Code Injection – Header
Link code: https://jsfiddle.net/qu38ao0r/
#3. Create a Dropdown
With Name/URL
#4. Click ADD PAGE
Ad add some Link items to Secondary Nav Dropdown
#5. Result