Question
Is it possible to add JavaScript inside a Rich(Floating) Interstitial message?
Answer
JavaScript cannot be added inside of the rich interstitial message template. The message template and values are inside a "Script" tag, so adding another one breaks it.
What you can consider as an option is to use a Web Interstitial message template where you can provide your own page that contains the content. One thing to keep in mind is that on Android JS is disabled by default for the Web Interstitial message template. To enable it you have to use the guidelines in the following documentation article:
https://docs.leanplum.com/docs/web-interstitial-message-1#edit