Trick is to put the https in the URL instead of the http in the htm file.
change the following URL from
<a href="itms-services://?action=download-manifest&url=http://server.com/IPA/abc.plist">Install My App</a>to
<a href="itms-services://?action=download-manifest&url=https://server.com/IPA/abc.plist">Install My App</a>
Bob