When retroactively adding devices onto your Apple Business/School Manager account, you will need to use Apple Configurator 2 to establish the initial connection.
Before we move forward with that process, we should create a Server in Apple Configurator 2. This only requires a few minor steps.
First, open Apple Configurator 2 and navigate to Apple Configurator 2 > Preferences:
Once here go to the Server tab:
Here we'll click the + button on the bottom-left corner. This will take us to the following screen:
You can enter any Name, it is for your reference. For the value of Host name or URL, you can find that url within your Addigy Portal.
In your Addigy Portal you will navigate to Add Devices > Select Policy (The Policy that has Automated Device Enrollment Configured), then copy the enrollment link:
Once you've filled out the information and click Next, you'll have successfully created your first Server in Apple Business Manager!
Now you're on your way to getting your iOS device into Supervised mode. Please reference Apple's official documentation for the rest of the walkthrough: https://support.apple.com/guide/apple-configurator-2/prepare-ios-and-ipados-devices-automatically-cadf4637b47e/2.10/mac/10.15