addWindow.html

<!DOCTYPE html>
<html lang="en">
    <head>
        <title>Add Item</title>
        <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/materialize/1.0.0/css/materialize.min.css">   
    </head>
    <body>
        <div class="container">
            <form>
                <div>
                    <label>Enter Item</label>
                    <input type="text" id="item" autofocus>
                </div>
                <a class="btn-floating btn-large waves-effect waves-light red"><i class="material-icons">add</i></a>
            </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>