Okay! What this part of config.json does is tell Browsersync to run on the proxyUrlhttp://localhost:3000/ when you run the command yarn start, and then forward requests to http://localhost:3000 to your local development URL (devUrl) and return the response to the browser.
Since http://localhost/mysite is where your local development server is hosting your site, that’s the value to which you should set devUrl. Update that in your config.json, and then run yarn start from inside your theme’s folder. (Run just yarn first if you haven’t yet, to install the necessary dependencies.)
Once yarn start is running, try opening up http://localhost:3000 in your browser, and hopefully it will be working!