You can add any code that needs to be added to the <Head> tag of your website as shown below:
1. In your website editor, go to the 'Settings' tab.
2. Scroll down to the section called 'Custom JS'.
3. Insert your code there. Then click 'Save', and 'Publish' at top-right.
Adding a code in 'Custom JS' section means that the code will be applied to all the pages of your website.