How to create dynamic (conditional) placeholder in React.js

So I have a submit form where the user needs to create a task by typing in a task name. I want it to be empty at the beginning and have a placeholder of “you must enter a task” when the user click add without entering anything. Now I can achieve it to display the placeholder but it’s either always there or I encounter unreachable code. I know how to clean the submission & return to the add function, just need to be able to display the placeholder conditionally. Here’s what my code looks like atm: