Filling the COMMENT field with data using REPLACE

I have a music collection comprising several thousand titles. I want to place redundant information into the empty COMMENT field of several hundred files using the REPLACE command. I have been unsucessful in my attempts.

As an example, I would like to place the name ELVIS into 200 selected files containing empty COMMENT fields.

Seems simple enough, but I have failed on many attempts, and am frustrated.

Yosemite Sam

The Replace action only works inside a single field.
If you want to get data from one field to another, use an action of the type "Format value" for the target field (here: Comment)
Enter as format string: Elvis

If you want to put the same data (like the constant string "Elvis") into a field, it is easier to use the extended tags dialogue for the selected files - or, as COMMENT is already in the tag panel: enter the data there and click save.
If you want to copy the (variable) data from one field, e.g. ARTIST to COMMENT, then use the action type as mentioned above.
The format string would then look like this:
%artist%

There is a description in the FAQs how to create an action: /t/967/1