The LPUART module from the KW41Z supports hardware flow control and is configured through the LPUARTx_MODIR register. Please refer to chapter 38.2.6 LPUART Modem IrDA Register in the reference manual for information on this functionality.
The current MCUXpresso SDK for KW41Z is based on KSDK 2.2 and the LPUART drivers for this version don't support hardware flow control. This functionality will be supported in KSDK 2.3.
Additionaly, you need to configure the pin mux to the corresponding LPUART RTS/CTS signals.