Can you give me (super) admin and (s)ftp access so I can take a look?
You can use the 'Confidential information' button in the forum editor to hide sensitive information.
I am sorry, but you are asking me to add special support for an extension that has been discontinued and is not available for J4.
If you are updating that extension yourself, I am sure you can find a way to make that extensions work with Content Templater.
Most probably Content Templater is setting the value of the hidden field correctly. You'll need to figure out how to make it update the preview image there too.