Hi Ivan. To use a powerbank in order to start the engine in case of battery failure or discharge you will need to find a way to connect the powerbank to the charging port below the seat (not inside, just in the downside of the tail: there is a rubber piece protecting the connection port)
That port is similar to the laptop's, you'll need to find an adapter (male) and a usb cable, manually join it and then you will be able to connect the charging port and the powerbank, charge the battery and start the engine.
Keep in mind that the most common powerbanks capacity are rated at 3,7V. This must be able to output 12V and the intensity must be enough to charge the bike's battery. I would go for a 20.000mAh at least.
Hope this helps