There is a configuration option Master station that you set. The master station is triggered when any other station is triggered so you can activate pumps/main valves/etc.
You can use the Master Off Adjust option to have the Master station shutoff up to a minute prior to the Watering Station; this gives your supply lines time to reach head pressure.
The Master On Adjust option you would want to set to a small delay; this ensures your station valve is always fully open before the main valve is popped open and reduces the initial pressure surge.
All 3 options if used together will accomplish what you want; I believe they have been implemented for use cases just like this.
If your relay or your master valve is 24v AC it will work fine as long as its not a giant 2A coil relay for motors; replace one or the other with a comparable one that can be triggered by OpenSprinkler directly, should you run into any troubles.
From the Manual:
Master station: Master is a station that turns on when other stations are running. It can be used on a water pump or main valve, such that the pump or valve will turn on automatically when any other station is open. The default value is 0, which means no master station. A value larger than 0 selects a master station: any stations from 1 to 8 can be assigned as a master station.
Master on adjust: adjust the time (0 to +60 seconds) that master will turn on when a station opens. For example, +5 means the master will turn on 5 seconds after a station opens. Default 0.
Master off adjust: adjusted the time (-60 to 60 seconds) that master will turn off when a station closes. For example, -5 means the master will turn off 5 seconds before a station closes. Default 0.