You would not use printf to just send data through the USART. You should use the interface defined by Harmony ([Page 1295][1]). If you are looking to send something akin to debug messages, there is a debugger console that is supported that uses the SYS_MESSAGE() function which might be similar enough. There is an example harmony project that demonstrates how to use it (located at microchip\harmony\v2_05\apps\examples\system\debug_uart\firmware\src)

  [1]: http://ww1.microchip.com/downloads/en/DeviceDoc/Driver%20Libraries%20Help%20v2.06.pdf