We recommend using https as it's more secure. The value highlighted is:
https://7b1fde64.ngrok.io
Navigate to sign.dropbox.com and log in to your development account. Hover over your email address in the upper right corner and click Settings > API.
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:
https://www.hellosign.com/home/myAccount#api
Place the ngrok.io url value into the Account Callback box, and click TEST. If your callback handler is setup correctly, you'll see the Success! message below: