If you're working with the WAB SDK like I am, I constantly run into the 400 Invalid redirect_url issue. Here's how I resolve it:
In AGO I create a new application. In the new UI select the 'other application' application type. Fill out the required fields and save it. If you've already done this, you can skip this step.
Next I go to the settings tab for my newly created application and scroll to the bottom. There's a button labeled 'Update'. Click it.
In the redirect URI field, I use the fully qualified domain name (http://mymachinename.domain.com:3344). If you don't know your machine name, you can open a command prompt and type in 'hostname' and hit ENTER. The 3344 port is the default port for WAB SDK. I also add a redirect URI for https connections as well (https://mymachinename.domain.com:3345). Adjust the ports appropriately if you're using different ones.
Click Update button.
In the WAB SDK folder, there is a file called signininfo.json. If you see it, delete it. Launch your WAB home page, enter the App ID and other required info, and you should be on your way to creating your next app.
HTH
