How to add a preview step and a message to be send to users when they have submitted their form?

Preview step : to check if all information are correct;

Message to be send to users when they have submitted their form : something like : "Your form was send with success." And other pertinent information to add...

Thanks!

From what I understand of your question here you're mainly looking for a preview to verify followed by a message send to the user as a confirmation workflow.

In short, yes both of these are possible.

(1) To add a preview step to your workflow simply add our preview widget to your form.

(2) As for sending the user a message I would like to point out that there is a thank you message which is displayed after your submission has sent which you can customize. However, if you want to send them one as well you can do that by setting up an autoresponder.