Insert a value in iFrame

Insert a value in iFrame

I've got this structure and I want to change the values of the form based
on the form before. Problem is, the iframe got no ID and I can't set one
as it is a plugin that retrieves the data from another domain.
...<input type="button"
onclick="document.getElementById('betterplace').style.display='block';
var betterplace =
document.getElementById('betterplace')[0];betterplace.getElementsByTagName('iframe')[0].document.getElementById('donation_presenter_address_first_name').value=document.getElementById('_vorname').value;
" style="border:none; height: 50px; width: 250px;
font-weight:bold; font-size: 14pt;
background-color:#94C119;color:white;cursor: pointer;"
value="Ihre Spende abschlie&szlig;en"/>
<div id="betterplace" style="display:none;">
<div
style="width:500px;height:2px;margin-bottom:5px;background-color:
#94c119"/>
<br/>
<script type="text/javascript">
...