Script for Roblox to weld uploaded model and add a root part

NormalizeModel.lua

local model = game.Selection:Get()[1]

local root = Instance.new("Part")
root.Name = "RootPart"
root.Size = Vector3.one
root.EnableFluidForces = false
root.CanCollide = false
root.CanQuery = false
root.CanTouch = false
root.Transparency = 1
root:PivotTo(model:GetPivot())
root.Parent = model
model.PrimaryPart = root

for _, part in pairs(model:GetChildren()) do
	local weld = Instance.new("WeldConstraint")
	weld.Parent = part
	weld.Part0 = part
	weld.Part1 = root
end