http://codepen.io/eond/pen/kztDK
html
<form name="form" id="form" method="post">
<select onchange="input();" id="sel" name="sel">
<option value="" selected="selected">은행</option>
<option value="대구은행 715-12-12423423 정낙훈">대구은행</option>
<option value="농협 758-12-156966 정낙훈">농협</option>
<option value="우리은행 054-332-9900">우리은행</option>
</select>
<input type="text" id="box" name="box" class="FORM1" size="20" value="" readonly="readonly" />
css
input{border:0;width:500px;text-align:right}
js
function input(){
var kim = document.getElementById("sel").value;
document.getElementById("box").value = kim;
}