Instantiate prefab as child at a transform? (Javascript/Uniyscript)

I am having a problem with Instantiating a prefab at a transform location. I asked a similar question yesterday about how to Instantiate prefab at a transform but the answer I got seems to require me to give the location of spawn with Vector3 and Quaternion. I am unsure on how to get these to for local space as I don’t want it to spawn somewhere random in the world.
Yesterdays Question

Current script:

var Weapon1 = Instantiate(GunType, Weapon1Spawn.transform.position, Weapon1Spawn.transform.rotation, Weapon1Spawn); 

You can also just set the parent manually after instantiating:

Weapon1.transform.parent = Weapon1Spawn.transform;

(I believe you can also use Weapon1.transform.SetParent)

I’m no expert, but maybe just start it non-active in that transform, and then when called, just activate it.

That is what I do for most of my projects.