Okay I found it.
We can set it from the dwData parameter in the SimConnect_TransmitClientEvent function
SimConnect_TransmitClientEvent(hSimConnect, 0, AP_ALT_INC_EVENT, 1000, SIMCONNECT_GROUP_PRIORITY_HIGHEST, SIMCONNECT_EVENT_FLAG_GROUPID_IS_PRIORITY);
But if it’s not set yet (showing _ _ _ _ on the PFD), the first event increase by 200. If anyone know why, I’ll take it