How to use a mesh in game, not a roblox asset?

I am currently creating a tycoon game with droppers, here is the code within a specific dropper:

wait(2)
workspace:WaitForChild("PartStorage")


deb = true 
script.Parent.Clicker.ClickDetector.MouseClick:connect(function(wat)
	if deb == true then
		deb = false
			local part = Instance.new("Part",workspace.PartStorage)
			part.BrickColor = script.Parent.Parent.Parent.DropColor.Value
			local cash = Instance.new("IntValue",part)
		cash.Name = "Cash"
		cash.Value = 3
		part.CFrame = script.Parent.Drop.CFrame - Vector3.new(0,1,0)
		local mesh = Instance.new("SpecialMesh", part)
		mesh.MeshType = "FileMesh"
		mesh.MeshId = "rbxassetid://57718450"
		mesh.TextureId = "rbxassetid://139677274"
		part.TopSurface = "Smooth"
		part.Size = Vector3.new(1,1,1)
		part.Rotation = Vector3.new(-0,-0,0)
		game.Debris:AddItem(part,20) 
			wait(.15)
			deb = true
	end
end)

My question is, how would I make the mesh.MeshId use a model that is within the game, not one that is taken from the roblox website (a roblox asset).

I would imagine it would look something like:

mesh.MeshId = script.Parent.DropModel

(But I know this is wrong) I am pretty new to scripting so if you could explain in detail it would be helpful, thanks!