UIKeyCommand external keyboard keys, extract from https://github.com/kishikawakatsumi/KeyboardShortcuts/blob/master/KeyCommands/ViewController.m

key-command.m

NSMutableArray *commands = [[NSMutableArray alloc] init];
    NSString *characters = @"`1234567890-=qwertyuiop[]asdfghjkl;'zxcvbnm,./";
    for (NSInteger i = 0; i < characters.length; i++) {
        NSString *input = [characters substringWithRange:NSMakeRange(i, 1)];
        
        /* Caps Lock */
        [commands addObject:[UIKeyCommand keyCommandWithInput:input modifierFlags:UIKeyModifierAlphaShift action:@selector(handleCommand:)]];
        /* Shift */
        [commands addObject:[UIKeyCommand keyCommandWithInput:input modifierFlags:UIKeyModifierShift action:@selector(handleCommand:)]];
        /* Control */
        [commands addObject:[UIKeyCommand keyCommandWithInput:input modifierFlags:UIKeyModifierControl action:@selector(handleCommand:)]];
        /* Option */
        [commands addObject:[UIKeyCommand keyCommandWithInput:input modifierFlags:UIKeyModifierAlternate action:@selector(handleCommand:)]];
        /* Command */
        [commands addObject:[UIKeyCommand keyCommandWithInput:input modifierFlags:UIKeyModifierCommand action:@selector(handleCommand:)]];
        /* Control + Option */
        [commands addObject:[UIKeyCommand keyCommandWithInput:input modifierFlags:UIKeyModifierControl | UIKeyModifierAlternate action:@selector(handleCommand:)]];
        /* Control + Command */
        [commands addObject:[UIKeyCommand keyCommandWithInput:input modifierFlags:UIKeyModifierControl | UIKeyModifierCommand action:@selector(handleCommand:)]];
        /* Option + Command */
        [commands addObject:[UIKeyCommand keyCommandWithInput:input modifierFlags:UIKeyModifierAlternate | UIKeyModifierCommand action:@selector(handleCommand:)]];
        /* Control + Option + Command */
        [commands addObject:[UIKeyCommand keyCommandWithInput:input modifierFlags:UIKeyModifierControl | UIKeyModifierAlternate | UIKeyModifierCommand action:@selector(handleCommand:)]];
        /* No modifier */
        [commands addObject:[UIKeyCommand keyCommandWithInput:input modifierFlags:kNilOptions action:@selector(handleCommand:)]];
    }
    /* Delete */
    [commands addObject:[UIKeyCommand keyCommandWithInput:@"\b" modifierFlags:kNilOptions action:@selector(handleCommand:)]];
    /* Tab */
    [commands addObject:[UIKeyCommand keyCommandWithInput:@"\t" modifierFlags:kNilOptions action:@selector(handleCommand:)]];
    /* Enter */
    [commands addObject:[UIKeyCommand keyCommandWithInput:@"\r" modifierFlags:kNilOptions action:@selector(handleCommand:)]];
    
    /* Up */
    [commands addObject:[UIKeyCommand keyCommandWithInput:UIKeyInputUpArrow modifierFlags:kNilOptions action:@selector(handleCommand:)]];
    /* Down */
    [commands addObject:[UIKeyCommand keyCommandWithInput:UIKeyInputDownArrow modifierFlags:kNilOptions action:@selector(handleCommand:)]];
    /* Left */
    [commands addObject:[UIKeyCommand keyCommandWithInput:UIKeyInputLeftArrow modifierFlags:kNilOptions action:@selector(handleCommand:)]];
    /* Right */
    [commands addObject:[UIKeyCommand keyCommandWithInput:UIKeyInputRightArrow modifierFlags:kNilOptions action:@selector(handleCommand:)]];
    /* Esc */
    [commands addObject:[UIKeyCommand keyCommandWithInput:UIKeyInputEscape modifierFlags:kNilOptions action:@selector(handleCommand:)]];

sample in Objective-c, if you need to use it with swift follow the sample bellow
/* Enter */
UIKeyCommand.init(input: "\r", modifierFlags: [], action: nil)