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 and click on Active Users. Note: do not go to groups > shared mailboxes as the password option is not available in that section). https://admin.microsoft.com/AdminPortal/Home#/users
Step3. Select the shared ‘unlicensed’ mailbox from the list. Hover over your shared mailbox and click the key icon that appears.
Alternatively, click on your shared mailbox and click “reset password” on the next screen.
Step 4. Press re-set password
Step 5. Specify a password.
Step6. Leaving the Office 365 portal in a new tab open up timetoreply and 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.