The vanilla quake way is this:

- put the monster in an enclosed box outside the level
- put a trigger_teleport around the monster
- point the teleport to an info_teleport_destination in the level
- give the trigger_teleport a targetname and target it from whatever you want (another trigger, a button press, a key, etc.)

But if you're making a mod (or using a mod) you can do it simply with spawnflags. Many mods have a "spawn_triggered" flag or similar, which makes the monster wait until triggered before spawning. Just place the monster where you want them to spawn, and then trigger them directly. If you're making your own mod you can try to copy this logic from an existing mod that has this feature. 

Simple mapping questions don't need their own thread. You can ask them in the Mapping Help Thread 

over in the discord :)