Hi rkovvur ,
What you are referring to as the “main valve” is know as the master valve in OpenSprinkler terminology.
You can hook up the master valve to any of the first 8 stations on the OS. Then go into options and select that station as the master.
You will also need to go to the Stations page. When a master has been selected, you will see a column of check boxes next to each station for activating the master.
On the options page you can also set an optional delay for the master.
Dan