The master zone cannot be activated directly, but it can be activated if any zone that associates the master zone turns on. So you can define a dummy zone (it can be any zone, including the ‘virtual’ zones that are not physically available), associate it with master, and using that zone to activate master.
OpenSprinkler supports program switch — you can use a switch to start program 1 — That program can contain only the dummy zone so when it’s on it only turns on the master zone. If program 1 is only meant to run manually through the switch, you can set it to be ‘disabled’ so it won’t run normally but will run upon pressing the program switch.