<form name='formname'>
<input type='text' name='inputname' />
<input type='button' value='button' onclick="window.open('popup.htm');" />
</form>

<form name='popupform'>
<input type='text' name='popuptext' />
<input type='button' value='button' onclick="window.opener.document.forms['formname'].elements['inputname'].value = document.forms['popupform'].elements['popuptext'].value;" />
</form>