I’m having the same issue, but with a different corporate TA.
I’ve written to them pointing out that:
1. The terms of their offer do not exclude agency bookings.
2. Their FAQs specifically outline how to offset a flight after booking and I relied upon this in my decision to offset my flight.
3. The confirmation message after I offset my flight, explicitly said I had completed one of the required milestones. I also attached a screenshot.
