While it’s not possible in TIME Sites to add code directly into the <head> section of your website, you can easily achieve the same thing by using an embed element and placing it at the top of the page.
Add a new embed element to your page by dragging it from the left toolbar:
Place the embed at the top of your page, above all other content. Then click the small embed icon in the center of the element to open the embed code window:
Paste your code into the window and make sure to uncheck iframed:
You will need to click "I understand" to allow the code to not be in an iframe.
Click Submit to add the code into the site.
Hiding the Embed
By default, the embed element will take up a bit of space at the top of the page. To prevent it from affecting your page design, you can use the Dimensions Panel to set its height to 0. This will essentially “hide” the embed on the page. (You can still select it in the design studio via the embed icon, or the Page Structure Panel.)