Hello YingXia Feng,
Thank you for posting this case in Microsoft Community!
Based on your description: How to set password of shared mailbox? I would like to cooperate with you working on this case. To clarify this case, shared mailbox does not need any password for authentication in Outlook which only requires grant the access permission for your primary email account. For my suggestion, you may check this article as reference Open and use a shared mailbox in Outlook. Generally, after your admin has added you as a member of a shared mailbox, close and then restart Outlook. The shared mailbox should automatically appear in your Folder pane in Outlook. (**Note:**Shared mailbox will be automatically synced and showing in your Outlook but you may wait for at least 1 hourfor sync process. )
By the way, if auto-sync process is not working well for shared mailbox, you also can try to manually add it under your primary account settings.
Hope the above workaround would be helpful. If you need further assistance, please feel free to let me know, I will keep assisting you.
Your understanding and patience will be appreciated.
Thank you!
Microsoft Community Moderator