I need a glossary/Secondary navigation list specific for one category of my primary navigation on the Rally template

Site URL: https://dewildesalhab.com/

I have the rally installed on my Squarespace website. I'm trying to make a kind of secondary navigation specific to one category of my primary navigation options. 

When people go into 'skin scandal' I want a secondary navigation to appear under the logo where they can choose items tagged under specific category (for example: serum. toner. mask.)

I'm guessing I need specific code injection and thank you in advance for your programming prowess