Try the following HTML code:
<form action="http://pdfcrowd.com/api/pdf/convert/uri/"
method="post">
<input type="hidden" name="username" value="USERNAME" />
<input type="hidden" name="key" value="APIKEY" />
URL: <input type="text" name="src" value="http://google.com"/><br />
<input type="submit" value="Create PDF" />
</form>
(Do not forget to replace USERNAME and APIKEY with your credentials)