Fill in with your account name:
<br><input type="text" id="accname"><br>
<input type="submit" onClick="return redirectToSignature();" value="Create signature!"><br>

<script>
    function redirectToSignature() {
        var accName = document.getElementById("accname").value;
            if ( accName ) {
                window.location = "signature.php?username=" + accName;
                return true;
                    }
            else        {
                alert ( "Please fill in the field." );
                    }
    }
</script>