I highly recommend re-reading the list of tags in the WC manual (pages 198 thru 212), as well as the pages covering how to use them and how WC does its 'magic' with them.

It's as simple as writing a plain text file, with nothing more than the WC tag, saving the plain text file as ".html" (or even ".htm") and adding it to the list in Preferences->On-Line->Custom Web->Additional Files. Of course, you have to set up the FTP info, but you don't even have to upload the file yourself. WC will see 'read' that list, open the file, look for WC tags and convert it to whatever it does (
GRHUMI$ tells WC you want a
graph of the
indoor humidity). Then, WC will even upload the edited file for you. All you have to do is type in the correct url (and you can even test it on your own computer). I've over simplified the description for my simple mind.
Of course, as a IT guy, you already know all the web/FTP/javascript/PHP/coding details. I think of WC tags as a 'markup language' that uses the app to convert that language into specifically formatted numbers, graphs, gauges, and charts. Since you're using a 'template', it may be more difficult to figure out where to place the tag, just depends on how flexible the 'template' is. Hopefully, it's as simple as adding "GRHUMI$" in a 'settings' file.