<select id="slList" onchange="selectShow()">
    <option value="100">A</option>
    <option value="200">B</option>
    <option value="300">C</option>
    <option value="400">D</option>
</select>
<input type="text" id="txtShow" />
<script>
window.onload=selectShow;
function selectShow(){
    document.getElementById("txtShow").value=document.getElementById("slList").value;
}
</script>