Firstly go to Trigger Editor by pressing
F4 or by clicking the giant
a button above.
Then click the New Trigger button above (or press
CTRL + T) to create a new trigger function.
After that, create a new Event by pressing the Flag button (or by pressing
CTRL + E on your keyboard).
Next, locate and find the
Time - Periodic Event function then press it and set your own periodic timer.
Lastly, create a new action by pressing
CTRL + R or through the Clip icon above, then find the
Unit - Create Unit Facing Angle action and set your own function calls there.
You're now done, of course be sure to make a region in where you want the units to spawn then set it to the action earlier. Well good luck and happy map making!