I see your site is on the WordPress.com Premium plan.
You can add custom CSS code to My Site > Customize > CSS. However, it’s not possible to add any other type of code, such as PHP or JS, into your theme or site on the Premium plan.
If that’s important to you, you may want to consider upgrading to the Business plan. You can install a plugin like Insert Headers & Footers or Code Snippets to add that type of code to your site on the Business plan.
Also just a heads-up that since your site has a Premium plan, you’re entitled to live chat and email support, accessible here: