Now we have a Azure B2C Tenant created – we need to add a new Application Registration into the Directory, this will act as the go between component from the Portal into the Azure B2C.
We do this within the Azure Portal, making sure we have Switched Tenants into our Azure B2C Tenant and then navigate into App Registrations.
This may be blank if we are using a new Azure B2C Tenant, but not to worry, we are going to add our first App into the Directory and specify the right details to link up with our Portal.
New Registration will open a screen where we can supply the Name and Redirect URL of our new App Registration.

The Redirect URL will be the URL of our Portal, but with a forward slash and then the name we have configured for Azure B2C Authentication, if we are using default settings then this will be /signin-b2c
https://mygreatbigportal.powerappsportals.com/signin-b2c
This App Registration is the connecting glue between the Directory and the Portal.
Access Tokens for Implicit Flow
Power Apps Portals uses an Access Token for authentication to allow an implicit flow from the Portal Login into Azure B2C – and this needs to be configured in our App Registration – this is done in the Authentication area of the App Registration, and then by ticking the ‘Access Tokens (used for implicit flows)'’ setting.

Application Client ID
Each Application we register will come with an auto-generated Client ID as the unique identifier for the App – we should note this down within Azure as we will need to configure this between the Site Settings of our Portal and Azure B2C:

This corresponds to the following setting in the Site Settings of our Portal Apps Portal configuration:

With this Site Setting configured between Portal Management and Azure B2C, this will inform how the Portal communicates to Azure B2C.
Now our Application Registration set, we then need to define the appearance and logic for how the flow of the User Authentication will work – we do this via User Flows in our Azure B2C Tenant.