Since you reach age 65 in October 2019 and have applied for Medicare, your Medicare coverage will begin October 1 2019. Since you will be covered by Medicare for the last 3 months of the year, if you have been covered by the HDHP for January through September your HSA contribution limit for 2019 is 9/12 of your regular contribution limit. If you are married and the HDHP plan includes coverage for your spouse who remains an HSA-eligible individual for the rest of the year, your spouse could contribute to your spouse's HSA the remainder of the family-coverage HSA contribution limit, plus your spouse's own catch-up if age 55 or over.
