I have not tried it by myself but looking into the schematics of the Linker Mezzanine card is converting 1.8V UART for the serial console with level shifter on connector "UART".
Pin1 - GND
Pin2 - VIN
Pin3 - TX
Pin4 - RX
The schematics is unclear whether it is converting to 3.3V or 5V UART, probably it is 3.3v, so please check the voltage with your multimeter first.
Please let me know if it is 3.3V.