Blue Pill Backup Domain RAM

[pwrgreg007]

I am in need of a few bytes of backup domain RAM on an STM32F103C8 Blue Pill project, and did not find any similar discussions on this forum. I am using the RTC with a coin cell battery backup and that works fine. I plan to use the alarm clock functions as well. I cannot find a diagram of which BKR RAM is used by the RTC and which is free for general use, in the RM0008 document. I understand that there are 40 bytes BKR total on this chip.
I am using the official STM32 core library. Anyone know of a library, sample code, or blogs discussing this?
Thanks in advance - Greg
Update: I found "AN2821 Application note Clock/calendar implementation on the STM32F10xxx microcontroller RTC" but it does not show the layout or addresses of this BKR memory. Confusing!