RTC backup battery

How can one add a coin cell battery to keep the RTC alive when power is removed from the MKR1000?

It looks like this should be possible based on the RTC library reference document (RTCZero - Arduino Reference). This document states that a coin cell battery can be connected to the 3.3V pin through a diode to keep the RTC alive. However, I don't see a 3.3V pin on MKR1000.

I thought the VCC was 3.3V (pin 12 I belive)

OOPS ignore me on that.

Sorry, I didn't ask that well. The ATSAMW25 runs from the 3.3V supply, so a battery connected there would supply the entire module rather than just the RTC, if I'm not mistaken. I don't see anything -- pin, test point, or otherwise -- that would supply just the RTC. Looking at the ATSAMW25 / SAM 21 datasheets, it's not obvious to me that powering just the RTC is possible. Am I overlooking something?