@Tianjiao
When you register an application through the Azure portal, an application object and service principal are automatically created in your home directory or tenant. You should be able to search your application when you add a role under Access Control(IAM) for the storage account. For more information you can refer to this page on creating an Azure AD application and service principal that can access resources.
Hope this helps! Let us know if you are still facing issues or have further questions.
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