Hi, Enzo.
You can use the presented schematic, BUT with certain modifications:
1. Connect the base of Q1 to X8 Key (pin 13) in ExtCtrl connector via resistor R1_10K.
2. Pull +12V from pin 14 and connect to R2 resistor in parallel with PTT line from EB104 PA.
In ExpertSDR2 settings, you can adjust PTT delay.
Just FYI on our website you can find this schematic -
eesdr.com/images/Document/EB300_to_ExtCtrl.pdf