To grant someone access to your mailbox, you need to adjust your folder permissions. After selecting your inbox, navigate to the Folder tab and click Folder Permissions. Add the user you want to grant access to, set the desired permission level, and click OK.
Once permissions are set, the other user can access your mailbox through File > Open > Other User’s Folder in Outlook or by right-clicking your name in the navigation pane in OWA. In OWA, the shared mailbox will appear in the navigation panel.
For detailed instructions, refer to the Microsoft article: Share and access another person's mailbox or folder in Outlook.