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 page: https://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.