How can I use ExifTool to prepend text to image files' descriptions?

[sdm]

I need to prepend text to the description fields of a bunch of jpgs. I found discussions of appending text (e.g., here and here), but not of prepending text. My destination tag is mwg:description, and my understanding is that the current value of that tag is $mwg:description, so I think I'm looking for something like this:

exiftool -mwg:description="Text to Prepend $mwg:description" *.jpg

but that just replaces the current description with Text to Prepend; the existing description text is lost. Can someone help me with the proper incantation?

[Phil Harvey]

This is common mistake 5c.

Also, you need to use single quotes if you are on Mac or Linux (or Windows Powershell).

- Phil