Some third-party services require you to insert code into the <head> section of your site rather than embedding a widget or form directly into a canvas (see Embedding Widgets for more info). Common examples for this include Facebook Pixel, Pinterest's Rich Pins, Online chat systems, or third-party tracking services (Excluding Google Analytics which has a built-in option. See: Setup Google Analytics).
Inside the Showit builder, select the page name in the top left corner. If a canvas is selected it will only show canvas settings and not the page settings. To avoid this, click on the page name.
In the right toolbar, you should see an a tab for "Advanced Settings". Select it and it will reveal a box for adding Custom Head HTML.
Clicking the box will pull up a pop-up window that will allow you to paste your code. Click "Save" when finished.