How to change a TextBox's text into asterisks, like a password textbox?

The number of characters in a string can be counted using #string, so we can use this to iterate through each character in the string, replace it with an asterisk, and repeat until we get to the end. After that, just replace the TextBox text with the string we just made. We'll use the FocusLost event to update the text.

TextBox = script.Parent
local replacer = "*"

TextBox.FocusLost:Connect(function()
    local string = TextBox.Text
    local newString = ""
    for i = 1, #string do
        newString = newString .. replacer
    end
    TextBox.Text = newString
end)

Strings wiki post

FocusLost event