Hi @Albrecht M ,
This is a feature related to the Azure AD registration service, which enables users to register their (personal) devices with an Azure AD tenant, and based on my research, the prompt cannot be hidden form Apps.
Please kindly understand here which mainly focus on general issues about Outlook desktop client, and know little about Azure AD. It is suggested for you to post a new thread with" Azure Active Directory "tag, people in that forum will give you more appropriate help.
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