Recently I created a shared mailbox in an Exchange Online (office 365) admin center (no on-premise) and while Outlook users using Windows were able to get the shared mailbox to show up, one Mac user was not able to get the mailbox to show up.
The Mac user was using the version of Outlook bundled with Microsoft Office 365.
Microsoft gives these instructions for Mac: https://support.microsoft.com/en-us/office/open-a-shared-folder-or-mailbox-in-outlook-for-mac-6ecc39c5-5577-4a1d-b18c-bbdc92972cb2
But they do not work because the traditional email address + authentication screen does not show up, instead, it directs you to sign in online. Shared mailboxes do not have passwords though and you cannot enter your personal details as verification.
After some research, I came across this page: https://support.microsoft.com/en-us/office/the-new-outlook-for-mac-6283be54-e74d-434e-babb-b70cefc77439 which at the time read as follows: