We'll use ngrok for our example, as it's relatively easy to setup.
Once you have ngrok up and running, it'll look like this:
The value highlighted is
Go to hellosign.com and login to your development account, then hover over your email address in the upper right corner and click Settings. Then click on the API Tab.
That's how you've retrieved your API Key, and this is where you'd setup your API Apps as well. It's this url if you're already logged in:
Place the ngrok.io url value into the Account Callback box, and hit the TEST button. If your callback handler is setup correctly, you'll see the Success! message below:
If you want to continue receiving callbacks to that URL, make sure to click SAVE at the bottom of the page.
If you received an error, keep in mind that the two things HelloSign looks for is a 200 response with the string "Hello API Event Received" somewhere in the body.