How to get URL for ArcGIS online portal?
Not sure exactly which client that is, but you'll need to work with your account manager to get an ArcGIS Online subscription, or get a trial. Then, you'll need to configure your subscription.
That looks like the registration dialog for Web AppBuilder for ArcGIS Developer Edition. As Jonathan Quinn indicated, you will need to get an ArcGIS Online organization or Portal for ArcGIS instance for Developer Edition to connect and sign into. You will also need to register Developer Edition with the organization and get an App ID. FYI, a tutorial:
ArcWatch | Configure Web AppBuilder for ArcGIS Developer Edition to Work with Your ArcGIS Online Organization
Hope this helps,
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