From https://github.com/onetransistor/MediaKeyboard [modified for simultaneous keys and release for stm32]

MediaKeyboard.cpp

#include "MediaKeyboard.h"

#if defined(USBCON)
#include "usbd_hid_consumer_if.h"

typedef struct
{
	uint8_t MODIFIER;
	uint8_t RESERVED;
	uint8_t KEY1; //X
	uint8_t KEY2; //Z
	uint8_t KEY3;
	uint8_t KEY4;
	uint8_t KEY5;
	uint8_t KEY6;
} keyboardReport;

keyboardReport kbreport = {0, 0, 0, 0, 0, 0, 0, 0};

MediaKeyboard_::MediaKeyboard_(void)
{
}

void MediaKeyboard_::begin(void)
{
	HID_Composite_Init(HID_CONSUMER);
}

void MediaKeyboard_::end(void)
{
	HID_Composite_DeInit(HID_CONSUMER);
}

void MediaKeyboard_::press(uint8_t mediaKey)
{
	if (mediaKey == KEY_X)
	{
		kbreport.KEY1 = KEY_X;
		HID_Composite_keyboard_sendReport((uint8_t *)&kbreport, sizeof(kbreport));
	}
	else if (mediaKey == KEY_Z)
	{
		kbreport.KEY2 = KEY_Z;
		HID_Composite_keyboard_sendReport((uint8_t *)&kbreport, sizeof(kbreport));
	}
}

void MediaKeyboard_::release(uint8_t mediaKey)
{
	if (mediaKey == KEY_X)
	{
		kbreport.KEY1 = 0x00;
		HID_Composite_keyboard_sendReport((uint8_t *)&kbreport, sizeof(kbreport));
	}
	else if (mediaKey == KEY_Z)
	{
		kbreport.KEY2 = 0x00;
		HID_Composite_keyboard_sendReport((uint8_t *)&kbreport, sizeof(kbreport));
	}
}

MediaKeyboard_ MediaKeyboard;

#endif

MediaKeyboard.h

#ifndef MEDIAKEYBOARD_H
#define MEDIAKEYBOARD_H

#include <Arduino.h>

#if !defined(USBCON) || !defined(USBD_USE_HID_COMPOSITE)

#error "USB HID not enabled! Select 'HID' in the 'Tools->USB interface' menu."

#else

#define VOLUME_UP 0xE9
#define VOLUME_DOWN 0xEA
#define VOLUME_MUTE 0xE2
#define MEDIA_PLAY_PAUSE 0xCD
#define MEDIA_STOP 0xB7
#define MEDIA_NEXT 0xB5
#define MEDIA_PREV 0xB6
#define KEY_X 0x1B
#define KEY_Z 0x1D

class MediaKeyboard_
{
private:
public:
	MediaKeyboard_(void);
	void begin(void);
	void end(void);

	void press(uint8_t mediaKey);
	void release(uint8_t mediaKey);
};
extern MediaKeyboard_ MediaKeyboard;

#endif
#endif