The Safe-start module can be found in the Teamwork category inside the module list (F5).
Once placed you can double click the module itself to access the settings.
Choose who gets affected by the safe start.
All units
Every unit on the map.
Synchronized units only
Every unit that has been synced to the module.
Groups of synchronized units
Every units group members that synced to the module.
Every unit on the units side
Every unit on the synced units side.
The duration that the safe-start will last.