If I have model in the workspace which includes a normal type script, and then I have for example screenGui with a text box in the StarterGui. Can I in some way change the properties of the text box with the script, or does it reguire that the script is in the starterGui itself.
You'd have to get a player then, their PlayerGui then, the ScreenGui Object that contains the Gui's(Frames and stuff) for tht player then, the Frame(s) or TextLabel(s) or TextButton(s), etc. For Example,
local frame = player.PlayerGui.ScreenGui:FindFirstChildWhichIsA("Frame")
Which would just set frame to a Frame in tht players ScreenGui, which is in their PlayerGui.