The FTHR board has both the MAX32620 and MAX77650 chips (microprocessor and PMIC respectively).
The MAX77650 has a pin called PWR_HLD, and this pin has to be held high to keep the PMIC ON which in turn keeps the micro ON.
This pin is connected to P2_2 of the micro. Adding the following lines of code keeps the board turned ON when powered by a battery:
DigitalOut pwr_hld(P2_2);
pwr_hld = 1;