To enable the UART on your Raspberry Pi to be used as host you need to configure it:
5 - interfacing options
P6 - serial
Would you like a login shell to be accessible over serial? Answer: 'No'
Would you like the serial port hardware to be enabled? Answer: 'Yes'
Reboot your Raspberry Pi.
To establish a UART-Connection to the Surface RT use
minicom -b 115200 -D /dev/ttyAMA0
minicom -b 115200 -D /dev/ttyS0
Further information: https://www.raspberrypi.org/documentation/configuration/uart.md