How to add a "Email sent" element under form after submitting?

I am making a mail form in html that submitts values to a php sendmail file. When I submitt the form Id like a little box to popup, Ive seen it on other websites but cannot figure out how to do it myself. My form looks like this :

<form>

       <input type="text" id="name" name="Name" placeholder="Namn" required /><br />
       <input type="text" id="email" name="TheEmail" placeholder="E-Mail" required /><br />
       <input type="text" id="phone" name="Phone" placeholder="Telefon" /><br required />
       <textarea id="message" name="Message" placeholder="Message" rows="8" cols="80" required>/textarea><br />
       <button class="form-button" type="submit" value="Skicka">Skicka</button>
</form>