Using classic Outlook for Windows in business environments
Hi @Jas Smit
Thank you for contacting the Microsoft Q&A forum.
From my research, you can add apps through All Apps in the Home ribbon when using classic Outlook.
If you’d like to add a custom app to Outlook, here are the steps you can follow:
Reference: Use add-ins in Outlook - Microsoft Support
For a tenant-wide deployment of add-ins, please ask your IT admin to add them through the Add-ins settings in the Microsoft 365 Admin Center. Here is the direct link to the settings: admin.microsoft.com/Adminportal/Home?#/Settings/AddIns
Once there: Choose Deploy Add-ins > Select Upload a custom add-in > Upload using either a manifest file or a URL
I hope this provides helpful insight. If you have any updates or need further assistance, feel free to reach out.
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