I've seen people doing this in many games, but I don't know how to make my own
Under the textbox properties; there is a thing called ClearTextOnFocus
https://developer.roblox.com/api-reference/property/TextBox/ClearTextOnFocus
What it does is; if it's set to true
; it will erase every text when you click on the textbox.
All you need to do is set it to false
.