So, it took me 2 days to figure it out how to do it, now is working and ready to collect Subscribers....
//<p class="has-text-white is-size-7-mobile is-size-7-tablet alert alert-success success-message " style="display:none;">SUBSCRIBED !.</p>
// <form class="field has-addons" id="contactForm">
// <div class="control form-control ">
//<input type="email" class="input is-small is-rounded form-control email" id="exampleEmail" placeholder="Enter Email" required>
// </div>
// <div class="control">
// <button class="button is-small is-rounded is-primary" type="submit">
// SUBSCRIBE
// </button>
// </div>
// </form>
var firebaseConfig = {
apiKey: "xxx",
authDomain: "xxx",
databaseURL: "https://xxxxxxx-xxxx.firebaseio.com",
projectId: "xxxxxxx-xxxx",
storageBucket: "xxxxxxx-xxxx.appspot.com",
messagingSenderId: "xxxxxxx",
};
firebase.initializeApp(firebaseConfig);
// Reference messages collection
var messagesRef = firebase.database().ref('contactformmessages');
//take all form values and pass it to the Firebase
$('#contactForm').submit(function(e) {
e.preventDefault();
var newMessageRef = messagesRef.push();
newMessageRef.set({
email: $('.email').val(),
});
$('.success-message').show();
$('#contactForm')[0].reset();
});