How do I call functions and get the return values from a popup?

I've been trying to put together a simple proof-of-concept for an idea I had to create an extension that could call some functions and get data from a content script running on a page.

I can't seem to figure out how to get the communication between the popup and the content script.

I keep coming across information that says to either use the underlying chrome.runtime in the content script or use plasmo's messaging, but whenever I try to do any kind of messaging in the content script, I get an error saying that chrome.runtime effectively doesn't exist (trying to access a property of a non-object).

Any ideas what I might be doing wrong?
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