You access O365 shared accounts by logging into Outlook webmail as yourself at https://www.office.com
Select the Outlook icon and then click on the avatar/profile picture in upper right side of the browser and select "Open another mailbox".
Type name of shared account, click on email address, and click on “Open” button.
The shared account should open in another browser tab.
Since the shared accounts don’t have passwords, they get the authorization from your email account in Outlook webmail.