How do I make an enemy spawn when the player enters a script area?

[KHShox]

////////////////////////////
// Run when entering map
void OnEnter()
{

}

////////////////////////////
// Run when leaving map
void OnLeave()
{

}

////////////////////////////
// Run first time starting map
void OnStart()
{
AddEntityCollideCallback("Player", "DoorClose", "CollideScript", true, 1);
}

void CollideScript(string &in asParent, string &in asChild, int alState)
{
SetSwingDoorClosed("castle_2", true, true);

AddEntityCollideCallback("Player", "DoorClose", "ScriptArea_2", true, 1);


GiveSanityDamage(2, true);
PlaySoundAtEntity("DoorClose", "react_scare.snt", "Player", 0, false);

AddEntityCollideCallback("Player", "Monster", "Spawn", true, 1);

void MonsterFunc1(string &in asParent, string &in asChild, int alState)
{
SetEntityActive("Fag", true);
}

There is my whole code after I added your stuff in it. But I got an error upon loading my map.