FeedBlitz

According to FeedBlitz, you can find this by “logging into FeedBlitz, go to your Newsletter Center, and click the HTML subscription form button on the right.”

Most likely, you’ll need to include
http://www.feedblitz.com/f/f.fbz?AddNewUserDirect
as the form action field,
EMAIL
as the e-mail field name, and
something like this for the hidden fields (it’ll vary as this is what tells feedblitz what list to subscribe the visitors:

<input name="EMAIL_" maxlength="64" type="hidden" size="25" value="">
<input name="EMAIL_ADDRESS" maxlength="64" type="hidden" size="25" value="">
<input name="FEEDID" type="hidden" value="UNIQUE VALUE">
<input name="PUBLISHER" type="hidden" value="UNIQUE VALUE">