You are migrating from classic WVD to AVD and looking for the best way to achieve it. You are also looking for information on the management API.
The API referenced in your post is the legacy API for WVD classic. It can’t be used for the current ARM-based version of AVD.
The App Group related API is documented here: https://learn.microsoft.com/en-us/rest/api/desktopvirtualization/application-groups
We also offer PowerShell modules to automate the migration from classic to the current model: https://learn.microsoft.com/en-us/azure/virtual-desktop/automatic-migration
Hope that helps.