If you have already installed digispark libraries you can skip this step, otherwise follow the tutorial   "Connecting and Programming your Digispark"

Link: https://digistump.com/wiki/digispark/tutorials/connecting

Download the modified library "DigiKeyboard_6keys.h" and copy it to the following folder/directory:

c:\Users\username\AppData\Roaming\Arduino15\packages\digistump\hardware\avr\1.6.5\libraries\DigisparkKeyboard\

Under Linux
/home/username/.arduino15/packages/digistump/hardware/avr/1.6.7/libraries/DigisparkKeyboard/

Whenever is necessary to use the modified version with 6 simultaneous keys, include the following library

#include "DigiKeyboard_6keys.h" 

Then you can send simultaneous keys by adding keystrokes to output buffer

// clear buffer by default
memset( DigiKeyboard.reportBuffer , 0, sizeof( DigiKeyboard.reportBuffer));

// Now add up to 6 keys to report
DigiKeyboard.reportBuffer[i++] = _Key_A;
DigiKeyboard.reportBuffer[i++] = _Key_B;
DigiKeyboard.reportBuffer[i++] = _Key_C;
DigiKeyboard.reportBuffer[i++] = _Key_D;
DigiKeyboard.reportBuffer[i++] = _Key_E;
DigiKeyboard.reportBuffer[i++] = _Key_F;

// Now set to send the keystrokes in the next USB interrupt
usbSetInterrupt( DigiKeyboard.reportBuffer, sizeof(DigiKeyboard.reportBuffer));
  while (!usbInterruptIsReady()) {
    usbPoll();
    _delay_ms(5);
  }

To release all the keys, simply send a clear buffer