I found the answer in my question from webdev-dan and Tch (I can't upvote them as I don't have the reputation, sorry!). I was trying to access the response after it was posted to console as opposed to parsing the data in my script then sending to console.
Here is what I did to get the element from the response I needed (successUrl).
Javascript
const result = await frui.getSession();
console.log(result);
console.log(result.payload);
console.log(result.payload.successUrl);
Response in console

I'll now set the result.payload.successUrl as a variable to use later on.