Customizing nav bar

Hi,

So I have my nav bar centered, but I want a button on top of my nav bar that says contents and when clicked the nav bar becomes visible right below it. Code wise I was thinking the functionality be someone like this demo except it will display horizontal not vertical. I understand the code and everything I am little confused on how to exactly make it work with squarespace's built in nav bar already. Anyone have any idea how to maybe go about this? I included an image to help show how I kind of want it to look after one clicks conte button.Thank you!

You need to use CSS to hide Nav, then use JavaScript to attach effect to button

When clicking on  button, JavaScript will add a class: show-on-click to button, then we use CSS to show up Nav.

It is time-consuming.

You may need to be mentally prepared to hire someone to do this, in case you don't get an answer in the next few days.

Yeah I understand how to trigger the visibility i think my main issue is solely getting the button above the nav bar. When I put code blocks it only effects the body but how exactly can i add code to the very top of the page? I just want to simply add a button and place it at the very top of the page and I can make the functionality myslef

use CSS position: fixed/absolute to move button to top

or use JavaScript to insert button to top