Check for Button press and Enter press

I made a expand form after button press, but now I want to add Enter key press too, but I dont know how. Maybe someone can help me!?

CODE

$(document).ready(
    function() {
    $(".form-fieldError").hide();
        $("#expand-form-button").click(function() {
            if( document.getElementById('sign_up_email').value === '' ){
                $(".form-fieldError").show(1);
            }else{
                $("#form-expand").fadeToggle();
                $("#expand-form-button").hide(1);
                document.getElementById("mc-embedded-subscribe-form").action ="http://ift.tt/2dw5eQV";
            }
        });
    });

Also I want to make 3 input field check if these are not empty.