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!