Why doesn't the textbox becomes focused when key pressed?

In my custom Chat GUI the Text Box in which the user will type whatever he/she wants to chat. I want to focus the text box when a key is pressed, but it says that Focused is not a valid member of TextBox. Please tell me how I can fix this.

Here is the script:

local userInput = game:GetService("UserInputService")

local function inputBegan(input, gameProcessed)
    if input.UserInputType == Enum.UserInputType.Keyboard then
        local key = input.KeyCode
        if key == Enum.KeyCode.Slash or key == Enum.KeyCode.BackSlash then
            script.Parent.Focused = true
        end
    end
end

userInput.InputBegan:Connect(inputBegan)

Thanks!