Here's how to add a shared mailbox to an existing Outlook account as a separate profile that you can switch between easily. 

To set your shared mailbox as the default account email address in Outlook, please consult this article.

Note: this function currently only works on Classic Outlook. 

1. Hold Shift while starting Outlook to access the profile picker.
   
   Note: You can also access the picker by selecting File > Account Settings > Change Profile.
   Outlook will restart and send you to the profile picker dialogue box.
   
   

On the profile picker dialogue box, select Options.

Note: Before selecting New, consider selecting the check box for "Prompt a profile to be used." If this is enabled, you will get a prompt every time you start Outlook to pick which profile you want to open. This is helpful when you have multiple profiles and need to switch between them. 

1. Select New.
   
2. Type a name for the new Outlook Profile and select OK. 
   
   
3. Once the new profile is created, you will need to add an email account to the profile. In the add account dialog, you can optionally add more accounts. 
   

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Set up a shared mailbox as the default address in Outlook for Windows