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In a very simple way, with CSS and some Javascript lines.
Look like the example in your image just act in CSS. Have fun!!
<style type="text/css">
#showDiv {
width:250px;
margin:auto;
position: absolute;
border: 3px solid #73AD21;
top: 44px;
left: 10px;
z-index:1;
background-color: #F5F5F5;
text-align: center;
padding:3px;
color: black;
font-family: Arial;
font-size: 13px;
cursor: point;
}
.fechar {
text-align: right;
margin-right: 10px;
font-size: 12px;
color: blue;
}
</style>
<script type='text/javascript'>
function show() {
showDiv.style.display='inline-block';
}
function hide(obj) {
var el = document.getElementById(obj);
el.style.display = 'none';
}
</script>
<img src="https://i.stack.imgur.com/sFkYI.png" id="link" onclick="show()">
<div style="display: none;" id="showDiv" onClick="hide('showDiv')">
<h3 style="background-color: #C0C0C0">CAIXA DE ENTRADA</h3>
<p>Você pode configurar esta janela com CSS.No momento o CSS é conforme mostrado abaixo</p>
<p>#showDiv {<br>
width:250px; <br>
margin:auto;<br>
...............<br>
}</p>
<form action="/unanswered" method="post">
<p>Registre-se ou faça <a href="#">log-in</a></p>
<p><input type="submit" value="Registrar-se" style="color: #000000; background-color: #FFFFFF"> </p>
</form>
<p class="fechar"><span style="cursor:pointer">fechar</span></p>
</div>

This can be done in N ways. Start trying to do and if questions arise regarding the code you can ask specific questions.
– Sam
@dvd, actually N+1 ways. You forgot the example of my answer! The property
widhtin Code Snippet does not work properly, you’ve noticed this?– user60252
Welcome João Vitor Batistella, some things you should know are: https://answall.com/help/mcve. plus this https://pt.meta.stackoverflow.com/questions/5483/manual-de-como-n%C3%83o-fazer-perguntas/5485#5485 and finally mark an answer as it accepts https://i.stack.Imgur.com/evLUR.png and why to mark how it accepts https://pt.meta.stackoverflow.com/questions/1078/como-e-por-que-aceitar-uma-resposta/1079#1079
– user60252