Can i copy a field to another?

hi
i have tried this

How to copy one field value to another field?

Example how to copy the content of the ARTIST field to the ALBUMARTIST field:

Create a new action in an existing action group or make a new action group.

Action type: Format value
Field: ALBUMARTIST
Format string: %artist%

i set like this

in short to copy the content of composer "moby" to comment
after the action , the composer field disapear and i have no comment
look before and after
before

after