The following steps will take you through identifying and selecting your Exchange Online Archive from the X1 Professional Client, and indexing its contents.
1) Select Tools>Options and navigate to the Outlook under the Email selection on the left.
2) Make sure to unselect the "Ignore Additional Mailboxes" selection under Outlook Options.
3) Choose the "Select Folders To Scan..." button to bring up your Outlook Mailbox(s), and expand to expose the Exchange Online folder name.
Please Note: Your Exchange Online items will populate in your index after your next scheduled indexing update. To populate immediately, choose the "Index Outlook Now" button from the menu in Step 1 to manually begin an indexing cycle.