There are two options.
Use the BULK ADD option.
Authenticating a shared ‘unlicensed mailbox’
Step1. Logon to Microsoft Office 365 as your tenant admin to https://portal.office.com/
Step2. Navigate to the ‘Users’ section. https://admin.microsoft.com/AdminPortal/Home#/users
Step3. Select the shared ‘unlicensed’ mailbox from the list.
Step4. Press re-set password
Step5. Specify a password.
Step6. Leaving the Office 365 portal in a new tab open up TimeToReplyand navigate to the agents pagehttps://portal.timetoreply.com/tools/agents
Step7. Press Add Agent/Add Another
Step8. Give the mailbox a friendly display name, specify the mailbox email address and Office 365 as the service type, and press send invite.
Step9. Navigate to the agents / mailboxes section under tools https://portal.timetoreply.com/tools/agents
Step10. Press view current authentication link
Step11. Visit this link and sign in as the shared mailbox using the details you created earlier.
Step12. Going back to your Office 365 portal select the shared mailbox and then select block sign in.
Select Block the user from signing in and then press Save
This completes how to authenticate a shared mailbox in office 365 as an agent.