Platform: atmelavr
==========================================================================
ID               MCU        Freq  Flash RAM    Name
---------------- ---------- ----- ----- ------ ---------------------------
itsybitsy32u4_3V ATMEGA32U4 8MHz  28KB  2.50KB Adafruit ItsyBitsy 3V/8MHz
itsybitsy32u4_5V ATMEGA32U4 16MHz 28KB  2.50KB Adafruit ItsyBitsy 5V/16MHz

#define INTERVAL (50)

void atcommand(char *cmd)
{
	long elapsed;
	Serial.println(cmd);
	Serial1.write(cmd);
	while (!Serial1.available());
	while (Serial1.available()) {
		Serial.write(Serial1.read());
		elapsed = 0;
		while (!Serial1.available()) {
			elapsed += INTERVAL;
			delay(INTERVAL);
			if (elapsed >= 500) break;
		}
		if (elapsed >= 500) break;
	}
	Serial.write(0x0a);
}

void setup() 
{
	Serial.begin(9600);
	while (!Serial);
	Serial1.begin(9600);
	atcommand("AT");
	atcommand("AT+VERSION");
	atcommand("AT+NAMEhc_06_keyboard");
	atcommand("AT+PIN5678");
	atcommand("AT+BAUD6");
}

void loop() {}

Arduino | ItsyBitsy
3V3     | 3V
GND     | GND
6(10)   | RST (reset)
11      | MOSI
12      | MISO
13      | SCK

Keyboard.cpp              KeyboardLayout_es_ES.cpp  Keyboard_es_ES.h
Keyboard.h                KeyboardLayout_fr_FR.cpp  Keyboard_fr_FR.h
KeyboardLayout.h          KeyboardLayout_it_IT.cpp  Keyboard_it_IT.h
KeyboardLayout_da_DK.cpp  KeyboardLayout_sv_SE.cpp  Keyboard_sv_SE.h
KeyboardLayout_de_DE.cpp  Keyboard_da_DK.h
KeyboardLayout_en_US.cpp  Keyboard_de_DE.h

#include <Keyboard.h>

char buf[32];

void setup()
{
	Serial1.begin(38400);
	Keyboard.begin();
}

void loop()
{
	byte key, key1, key2, key3;
	while(Serial1.available()) {
		key  = 0x00;
		key1 = Serial1.read(); delay(10);
		key2 = Serial1.read(); delay(10);
		key3 = Serial1.read();
		switch(key1) {
			case 0x0d: key = 0x0a; break;
			case 0x1b:
				key = KEY_ESC;
				if (key2==0x5b) {
					switch(key3) {
						case 0x41: key = KEY_UP_ARROW;    break;
						case 0x42: key = KEY_DOWN_ARROW;  break;
						case 0x43: key = KEY_RIGHT_ARROW; break;
						case 0x44: key = KEY_LEFT_ARROW;  break;
					}
				}
				break;
			case 0x01: // ネットワーク設定
				Keyboard.write("sudo vi /etc/netplan/50-cloud-init.yaml",39);
				Serial1.println("sudo vi /etc/netplan/50-cloud-init.yaml");
				break; 
			default: key = key1; break;
		}
		if (key) Keyboard.write(key);
		sprintf(buf,"[0x%02x] 0x%02x:0x%02x:0x%02x\r\n", key, key1, key2, key3);
		Serial1.print(buf);
	}
	delay(10);
}

// Misc keys
#define KEY_UP_ARROW      0xDA
#define KEY_DOWN_ARROW    0xD9
#define KEY_LEFT_ARROW    0xD8
#define KEY_RIGHT_ARROW   0xD7
#define KEY_BACKSPACE     0xB2
#define KEY_TAB           0xB3
#define KEY_RETURN        0xB0
#define KEY_MENU          0xED
#define KEY_ESC           0xB1
........

byte key, key1, key2, key3;
while(Serial1.available()) {
　key  = 0x00;
　key1 = Serial1.read(); delay(10);
　key2 = Serial1.read(); delay(10);
　key3 = Serial1.read();
　switch(key1) {
　　case 0x0d: key = 0x0a; break;
　　case 0x1b:
　　　key = KEY_ESC;
　　　if (key2==0x5b) {
　　　　switch(key3) {
　　　　　case 0x41: key = KEY_UP_ARROW;    break;
　　　　　case 0x42: key = KEY_DOWN_ARROW;  break;
　　　　　case 0x43: key = KEY_RIGHT_ARROW; break;
　　　　　case 0x44: key = KEY_LEFT_ARROW;  break;
　　　　}
　　　}
　　　break;
　　default: key = key1; break;
　}
　if (key) Keyboard.write(key);
}