add.js

// pages/add.js

export default function Add() {

    const storeAddTask = useGlobalStore((state) => state.addTask);

    const handleSubmit = () => {
        storeAddTask({
          title: title,
          description: description,
          isDone: isDone,
        });
        router.push("/");
    }

    // ...
}

// Your final file should look like this:
// https://github.com/duplxey/back4app-containers-nextjs/blob/master/pages/add.js