JSFiddle - React, Tailwind, and code Playground

HTML

<a href="">Основы html<span class="mli" id="adm">20</span></a>

<a href="">Java Script<span class="mli" id="bfg">11</span></a>

<form name="test" method="post" action="input1.php">

  <input type='hidden' id='sum'/>
  
  <button id='button' value='Отправить на сервер'>Отправить на сервер</button>
  
</form>

JavaScript

var sumNum = +(adm.innerHTML)+(+(bfg.innerHTML));

sum.value = sumNum;