JSFiddle - React, Tailwind, and code Playground

HTML

<li><span class='add-on'></span>" .
    "<input class='' type='text' value='' />
    <input class='picker' type='hidden' value='' name=''  />
    <input class='datepicker' type='text' />
</li>
<p></p>

CSS

li > span + input {
    background: #dddddd;
}

JavaScript

$(document).on('keyup', function() {
    var getDate = $(".datepicker").val();
    var getMark = $("li > span + input").val();
    var plusVal = getDate + getMark;
    
    $(".picker").val(plusVal);
    $('p').text($(".picker").val());
});