What day did you start Medicare? On September 1st? If so, you would mark "Medicare" or "None" depending on the screen for September, October, November, and December.
If you started Medicare after September 1st, then you will mark October, November, and December as Medicare or None.
Are you still under the Family HDHP plan? If so, your spouse would report Family coverage all year if your spouse has an HSA. Of course, your spouse will report "Medicare" or "None" when he/she goes on Medicare in 2019.