Basically the haya 6.3 is a rebranded heng long 6.1 so should be exactly the same.
What you will need is a single channel 5V relay.
loop the microswitch cn6 a shorter wire would be better

The xenon flash has 3 wires red positive + , blue negative - and black positive signal. Remove the 3 pins from the plug they are in and put the red and blue wire into a 2 pin plug and plug into cn2 red on the left.

The black wire will go into com on the 5v relay i'm using a dual channel relays as i cant find my single 1

To provide the power for the positive signal take a wire from led 1 + and plug into NO on the 5v relay

To power and provide signal for the 5V relay you can use cn10 as the bb gun is no longer in use and is activated when you fire the gun .The negative wire will need to be looped to the ground and signal on the 5V relay and the positive will go to positive on the relay
So when you fire the cannon cn10 will activate the relay which will connect COM with NO sending a positive signal to the xenon flash