How do I add something before an id in a sound?

I'm making a developer product and I can't get something before the id of the sound . My script is:

script.Parent.MouseButton1Click:Connect(function()
	
	
	local mpService = game:getService("MarketplaceService")
    mpService:PromptProductPurchase(game.Players.LocalPlayer, 49078483)
	game.Workspace.Songid.Sound.SoundId = game.Players.LocalPlayer.PlayerGui.PaytoPlay.TextBox.Text 
    


end)

However I need to add this before the textbox text:

rbxassetid://

Is there any way to put this before the sound id the player puts in? Any clues would be appreciated!