How do I report COBRA premium payments when I received the subsidy?

If you are itemizing your deductions, then you would report your COBRA premiums after the subsidy.  If you include the subsidized amount, you would be getting a deduction for premiums you didn't pay or were reimbursed for. 

Thanks. I just wanted to make sure I didn't have to split it up - report the gross premium in one place, then the offsetting subsidy in another place.

You can do it either way.  You can report the net amount you paid, or you can report the full amount then when it asks for any medical reimbursements you can enter the subsidy. Either way, it will end in the same result on your itemized expenses.