First, when you grant full access on Exchange Admin Centre, it will automatically show the mailbox on user Outlook folder list
Admin > Exchange > Recipients > Mailboxes [Edit] > Mailbox Delegation > Full Access [Add user]
Under Outlook Folder list, after grant the full access permission, close the outlook and open it again, it will automatically show in the folder list and start to sync the email.
When adding the mailbox you need sync all the mail on outlook, it may take a few hours or half day for sync the email, it will affect the user receive an email, during this operation, should let user keep open the outlook for sync the email and user should work on Webmail.
Webmail display shared mailbox
Under Webmail, the full access will not show in folder list
You can just use “Open another mailbox” for access shared mailbox, it will open a new tab on browser for access Shared mailbox
If you would like the shared mailbox keep show in your webmail folder list…
Folder [Right Click Mouse] > Add shared folder > [Enter] shared mailbox name
The shared mailbox will show in Webmail folder