Вы должны Concat значение для предыдущего значения и заменить его на InputBox. var prv_val,f_val; $('#test').change(function() { var new_val = $(this).val(); if(new_val != ""){ prv_val = $('#target_input').val(); if(prv_val != ""){ f_val = prv_val + "," + new_val; } else { f_val = new_val; } $('#target_input').val(f_val); } });
#target_input {
display: block;
height: 30px;
margin-top: 20px;
width : 100%;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select id='test'>
<option value=""></option>
<option value="one@gamil.com">one@gamil.com</option>
<option value="two@gamil.com">two@gamil.com</option>
<option value="three@gamil.com">three@gamil.com</option>
</select>
<input value="" id='target_input'>
<form action="/action_page.php">
<select name="cars" multiple>
<option value="volvo">Volvo</option>
<option value="saab">Saab</option>
<option value="opel">Opel</option>
<option value="audi">Audi</option>
</select>
<input type="submit">
</form>