addWindow.html

<!DOCTYPE html>
<html lang="en">
<head> 
    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/materialize/0.100.2/css/materialize.min.css">
    <title>Add Item</title>
</head>
<body>
    <div class="container">
    <form>
        <div>
            <label>Enter Item</label>
            <input type="text" id="item" autofocus>
            <button type="submit" class="btn waves-effect waves-light">add Item</button>
        </div>
    </form>
    <div>

    <script>
        const electron = require('electron');
        const {ipcRenderer} = electron;

        const form = document.querySelector('form');
        form.addEventListener('submit', submitForm);

        function submitForm(e){
            e.preventDefault();
            const item = document.querySelector('#item').value;
            ipcRenderer.send('item:add', item);
        }
    </script>
</body>
</html>