Spawning containers, other palceables and creatures: it's all the same :) Check this tutorial:
http://www.lucasforums.com/showthread.php?t=143536). You'll find various script samples to do that.
Basically, all you need to spawn a creature is this:
void main()
{
//this line checks is the creature already exists to avoid duplicate objects.
//If your dialogue only runs once, you don't need this line. You can also use a local variable to do this.
if (!GetIsObjectValid(GetObjectByTag("creature_tag")))
//this line spawns the creature
CreateObject(OBJECT_TYPE_CREATURE, "creature_templateresref", Location(Vector(0.00,0.00,0.00), 0.0));
}
Replace the "creature tag" and "creature_templateresref by those corresponding to your placeable. Note that the template resref must not exceed 16 chars (contrary to the above example).
Finally replace the 0.00 by the x,y,z and orientation coordinates you want. If you don't want to bother with specific coordinates, you can also simply spawn the creature near your PC by replacing the last line of the script by this one:
CreateObject( OBJECT_TYPE_CREATURE, "creature_template", GetLocation(GetFirstPC()) );
Edit: I first posted a script to spawn a container instead of a creature as I misread your initial post. To spawn a container, the only thing you need to change is the OBJECT_TYPE_CREATURE ->it has to be OBJECT_TYPE_PLACEABLE.