JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="hiddenfieldval">
<input type="text" id="hiddenfieldtext">
    <select id="myselect">
        <option value="1">строка 1</option>
        <option value="2">строка 2</option>
        <option value="3">строка 3</option>
    </select>

JavaScript

document.querySelector('#myselect').onchange = function(){
    var els = this.getElementsByTagName('OPTION');
    for(var i = 0; i < els.length; i++){
        if(els[i].selected){
            document.querySelector('#hiddenfieldval').value += els[i].value;
            document.querySelector('#hiddenfieldtext').value += els[i].innerHTML + ' ';
        }
    }
}