If you are planning to do development using the Live Services you will probably want to register a Live Services Project where the Return URL is an address on you local machine. Unfortunately the UI on the Azure Services Developer Portal isn’t that informative which can lead to a bit of confusion. In the following image I’ve attempted to register a new project where the Return URL is a local address on my machine. Since the Domain is the top level domain that is in the Return URL I figured that I needed to put localhost in the Domain field.
As you can see this generated the error “Invalid application domain name.” Whilst this error message is technically correct it would be better if it said “you don’t need to specify a domain if your return url is on your local machine”. The upshot is that if you want to use a local address you can simply omit this field.