When I have a required field in a form, there is several ways to indicate to the user that this field is required. In addition, we can add client validation on that field, to force the user to fill this field.
But how can I indicate that a field is recommended? I don't like that idea to make this field required, but I would like to indicate to the user that adding an information in that field will add value to his inputs.
Of course, I can display a popup when the user submits the form saying that "You did not fill the information XXX. Are you sure you want to continue?", but I think it's too intrusive.
Do you have any better idea?