social/website_mass_mailing_name/templates/snippets.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2016-2017 Jairo Llopis <jairo.llopis@tecnativa.com>
     License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl). -->

<odoo>

<template id="s_newsletter_subscribe_form"
          inherit_id="website_mass_mailing.s_newsletter_subscribe_form">
    <xpath expr="//input[@class='js_subscribe_email form-control']"
           position="before">
        <input
            name="name"
            class="js_subscribe_name form-control"
            placeholder="your name..."/>
    </xpath>
</template>

</odoo>