Edit in JSFiddle

<div id="conferma" hidden>
    Sei sicuro di voler inviare il form?<br>
    <button id="si">Si</button>
    <button id="no">No</button>
</div>
    
<form action="asd.php" id="mioform">
    <input type="submit">
</form>

var mioform = document.querySelector("#mioform"),
    si = document.querySelector("#si"),
    conferma = document.querySelector("#conferma");

mioform.addEventListener("submit", function (event) {
    event.preventDefault();
    conferma.hidden = false;
});

[].forEach.call(document.querySelectorAll("#si, #no"), function (elm) {
    elm.addEventListener("click", function () {
        conferma.hidden = true;
    });
});

si.addEventListener("click", function () {
    document.querySelector("#mioform").submit();
});

html, body {
    height: 100%;
    padding: 0px;
    margin: 0px;
}

#conferma {
    position: fixed;
    z-index: 3;
    width: 150px;
    height: 150px;
    padding: 10px;
    box-sizing: border-box;
    border: 1px solid gray;
    background: white;
    left: 50%;
    top: 50%;
    margin: -75px;
}