I am working on deploying Snap-Ins chat to our website, including within our web-app and I want to be able to pass values from our logged in users into pre-chat fields in Live Agent. I found this documentation and pasted the code from the article in as a test in our sandbox, but when I do that the chat button goes away, so I can't even get to the pre-chat form.

Help article: https://developer.salesforce.com/docs/atlas.en-us.snapins_web_dev.meta/snapins_web_dev/snapins_web_populate_prechat.htm

All I'm doing is going to the chat button code, un-commenting the embedded_svc.settings.prepopulatedPrechatFields" line, and pasting in the following below it:
embedded_svc.settings.prepopulatedPrechatFields = {
FirstName: “John”,
LastName: “Doe”,
Email: “john.doe@salesforce.com”,
Subject: “Hello” };

Any help is much appreciated.