ATMEGA 4809 PF with external crystal @ 16 MHz

Other Hardware 3rd Party Boards
14

The correct statements would be:

protectedWrite(CLKCTRL.MCLKCTRLA, CLKCTRL_CLKSEL_XOSC32K_gc);  // Select crystal oscillator
protectedWrite(CLKCTRL.MCLKCTRLB, 0); //  Disable prescaler

iom4809.h doesn’t support Clock Out, but you might use

protectedWrite(CLKCTRL.MCLKCTRLA, CLKCTRL.MCLKCTRLA | 0x80); // Activate CLKOUT

Combined it would make

protectedWrite(CLKCTRL.MCLKCTRLA, CLKCTRL_CLKSEL_XOSC32K_gc | 0x80); // Crystal oscillator and CLKOUT