BTW requests is for requesting stuff. Not trigger stuff.
Edit: to answer your question anyway...
Create a region.
Go to trigger editor.
Make an event : Time periodic event. (OR) Time - Timer expires... Timer expires is a 1 time only event, periodic event continues to do things.
Change the time from 2secs to anything you want.
Create an action : Unit - Create units facing angle.
Change any of these underlined:
Create 1 footman for Player 1 (red) at (Center of playable map area)) facing default building facing degrees.
If you don't know what to do there, change "(Center of playable map area))" to the region you have created. And change footman to the unit you wish to be created.
Tada! Units will now spawn where you have created your region.