You need to have already installed Ebsta Inbox into Outlook and connected it to your Salesforce account. If you haven't already done this, refer to this article:
Connect your Outlook mailbox to Ebsta
1. In Outlook, click the Ebsta button in the Home menu to open the Ebsta side panel.
If you are using Outlook online, click New Message and at the bottom of the compose pane, click the Ebsta button to open the Ebsta side panel.
2. Click Connect with OAuth.
3. Click Allow.
4. Sign in to your Microsoft Account with your username and password.
5. Your mailbox will now be connected to Ebsta.
Note: To keep the Ebsta side panel open, click the pin in the top right corner of the panel.