There are various ways. The more secure is changing it at the template level or include in the template some links to external JS files that you store in a controlled place.
Bu if you really want to add some JS directly in a guided LP, this post might help: Best way to add a script in a guided landing page
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But that doesn't work for Munchkin.
The way to manage Munchkin is here: https://blog.teknkl.com/conditionally-loading-munchkin-on-marketo-lps-for-gdpr-related-policies/