how to set password for shared mailbox?

Do not set or distribute a password for the shared mailbox; keep its account blocked from direct sign-in, grant each user access, and have them add it through their own account in Outlook for iOS or Android. A shared mailbox has a system-generated unknown password that is not intended for use. ^1^

Administrator setup

From a Windows PC:

1. Sign in to the Microsoft 365 admin center using an account with the Exchange administrator role.
2. Go to Teams & Groups > Shared mailboxes.
3. Select the shared mailbox.
4. Under Members, select Edit, add each person who needs access, and save.
5. If explicit permissions are needed, open the Exchange admin center and go to: Recipients > Mailboxes > select the shared mailbox > Mailbox permissions > Manage mailbox delegation.
6. Grant each user:
   • Full Access — required to open and manage the mailbox.
   • Send As — required if mail should appear to come directly from the shared mailbox.
7. Save the changes. Microsoft documents both permissions as required for full shared-mailbox operation. ^2^

Do not unblock sign-in or reset the password on the shared mailbox’s corresponding user account. New shared mailboxes have sign-in blocked by default, and Microsoft says it should remain blocked. ^3^

Each person accessing it must:

• Be an internal user in the same organization.
• Have their own licensed Exchange Online mailbox.
• Sign in using their own account and authentication methods, including MFA if required.

The shared mailbox normally needs no separate license, although licensing is required for certain scenarios such as storage above 50 GB, archiving, or litigation hold. ^1^

User setup on iOS or Android

Shared-mailbox support requires both the user’s mailbox and the shared mailbox to be in Exchange Online. In the Microsoft Outlook mobile app:

1. Sign in to Outlook with the user’s own work account—not the shared-mailbox address.
2. Open the left navigation pane and tap Add Account.
3. Tap Add a Shared Mailbox.
4. If Outlook contains multiple accounts, select the account that was granted permission.
5. Enter the shared mailbox email address.
6. Complete the setup; the mailbox should then appear in the account list.

No shared-mailbox password is requested because access is authorized through the signed-in delegate account. ^4^

The shared mailbox can only be added to the Outlook for iOS or Outlook for Android app using this supported mobile workflow; it should not be configured as an independent account with shared credentials. ^2^

If users cannot add it

Check these conditions without enabling direct sign-in:

• The user has Full Access permission.
• The user’s own mailbox has an Exchange Online license.
• Both mailboxes are hosted in Exchange Online.
• The user selected Add a Shared Mailbox, rather than Add Email Account.
• The user signed in with the same organizational account to which access was granted.

For another person’s mailbox, Microsoft likewise requires Full Access and instructs users to add it through Add Account > Add a Shared Mailbox. ^5^

References

1. About shared mailboxes in Microsoft 365
2. Create a shared mailbox
3. Shared mailboxes in Exchange Online
4. Add a shared mailbox to Outlook mobile | Microsoft Support
5. I am a delegate. How do I add that mailbox to my mobile device? | Microsoft Support