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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