In Showit, with a page selected (only a page, setting will not be available if a canvas on the page is selected) you will see the "Advanced Settings" tab in the Properties Panel on the right. There you will find advanced options to add snippets of code to your Showit site. We have two options when it comes to Javascript:
Inline Javascript & Page Loaded Javascript
Inline Javascript will execute the code as the page loads. Ideally this would be used for functions that do not target specific interactions or elements on the page such as a button click. Inline Javascript would be used for functionality associated with tracking codes or something similar.
Page Loaded Javascript will execute the code after the page has loaded. This would be used in the context of the code targeting certain interactions or elements on the page after those have had the opportunity to load.
As a quick note with both of these fields, by default, they wrap the code in <script> tags so inputting your own <script> tags might cause issues with executing your code correctly.
If you are looking to embed code in the <body>, you can add it to any canvas through an embed widget following the steps here - How to Embed HTML Code.