Not possible, that capability was removed from OWA. See for work-arounds:
https://support.microsoft.com/en-us/office/connect-email-accounts-in-outlook-on-the-web-microsoft-365-d7012ff0-924f-4f78-8aca-c3912d886c4d
Outlook Web App 365
Sven Schulte-Hillen
21
Reputation points
Using Outlook 365 in the Web Ppp, can I access different accounts at the same time? I am using different domains and different emails all hosted at Microsoft. But would need the Web App to operate like my Outlook locally, so taht I have all my different accounts in one single platform. Is that douable?
Accepted answer
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Andy David - MVP 148.7K Reputation points MVP
2021-03-16T11:51:38.16+00:00
1 additional answer
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Sven Schulte-Hillen 21 Reputation points
2021-03-16T14:24:00.55+00:00 Wow, my firt experience in posting a question. Thanks alot for the quick answer. Had been tortuing my brain to find the right button for such an obvious task.