Counter for number of characters in a multi-line field

Counter for number of characters in a multi-line field

Hi, how do I add a counter to show the number of characters in a multi-line field in real-time, meaning, as the user types into it. I'm using Length function as a formula field but the problem with it is the user needs to move out of the multi-line field in question for the counter to update the value. Hence looking for a more ideal solution.