Sending, receiving, and organizing email in Outlook.com
For anyone discovering this thread in the future, I found the issue - you need to right-click "Folders" NOT your inbox, the select "Add shared folder or mailbox".
The instructions here are out of date: https://support.microsoft.com/en-us/office/open-and-use-a-shared-mailbox-in-outlook-d94a8e9e-21f1-4240-808b-de9c9c088afd#articleFooterSupportBridge=communityBridge
I found the answer here: https://answers.microsoft.com/en-us/msoffice/forum/all/add-shared-folder-option-missing-from-owa/e0ff30cd-b861-452e-b0d9-d12e2622c15d