If input field is empty fill it with random number

HTML

<header>
    <nav>
        <ul>
            <li><a href="home.html">Home</a></li>
            <li>
                <a href="#">Maintenance <span class="caret"></span></a>
                <div>
                    <ul>
                        <li><a href="?goto=country">Countries</a></li>
                        <li><a href="?goto=province">Provinces</a></li>
                        <li><a href="?goto=mobile">Mobile</a></li>
                        <li><a href="?goto=customer">Customer</a></li>
                    </ul>
                </div>
            </li>
            <li><a href="?goto=logout">Log Out</a></li>
        </ul>
    </nav>
</header>

CSS

header{
            background: #fee;
                height: 120px;
         border-bottom: 5px solid #f53301;
    -webkit-box-shadow: 0 12px 16px -6px black;
       -moz-box-shadow: 0 12px 16px -6px black;
            box-shadow: 0 12px 16px -6px black;
}