You can run reports in the MessageMedia web portal on messages sent via ActiveCampaign by utilising the Meta Data filter in the Detailed Reporting feature
Go to Reports and then Detailed reports, and in the Meta Data fields set the date range for the period for which you wish to run the report.
Next, set the Meta Data key as "Extension" and the Meta Data value as "ActiveCampaign":
Click on Apply filters and you will then see the report for message sent via ActiveCampaign within the selected date range.