JSFiddle - React, Tailwind, and code Playground

HTML

<form name='focusform' id='focusform'>
<textarea class='formular_550' NAME='myName' rows=6 id='textbaustein'></textarea>


<select class='chosen' style="width:554px;" name="recent_select" onChange="javascript:pushTXT();">

<option value="" selected>Textbaustein(e) einfĂĽgen...</option>
<option value="test 12345">Test 12354</option>
</select>  
</form>

JavaScript

function pushTXT ()
{

alert(document.focusform.textbaustein.value);

document.focusform.textbaustein.value += document.focusform.recent_select.value;
}