Event - A unit come within range. Help me

How do i create a event-unit come within range of [Unit not yet been create] for a unit that i haven't create (not in game yet)
Example: Like AoS. If u choose a unit it will create the unit that u choosed. So like if the unit is a casting unit. What variables should i need?

I tried Units, Unit-types but non worked!
And i found out a bew action: "trigger-add new event"
But i don't know how it works, so i really hope someone will help me...

Hope to get the answer/reply soon, thanks.

so what are you trying to do exactly?

I want my hero to cast a Mass web ability.
And i want this effect something like when u casted Locust Swarm. What i mean is, if i can that Mass web ability, the hero can still move (i've done this part).
But i want enemy units come within whenever the hero goes to.
Order my dummy unit to cast Mass web.
And it last 20sec.

So between this 20sec.
If enemy unit come within range of that casting unit (hero that casts Mass web)
My dummy unit will cast web at enemy units.

BUT MAIN PROBLEM IS:
That event can add variables...
So how am i going to do this?

Description of the spell: Creates a webbing force around the spider for 20sec, causing him to release webs to any enemy units around him. The webbings lasts 25 seconds, dealing 20 damage per second.

you could use "unit comes within range of casting unit" or you could add the unit to a variable and use it (not sure if it will work) the casting unit should work fine though, unless its a skill used by multiple units at once.

After creating the unit use the

Trigger Add event action.

if you set up a trigger

e: unit is created
c: unit is equal to herotype
a: add event to triggerwhatever (unit(lastcreated unit) begins casting ability)

e: none originally
c: ability cast = massweb
a: your actions