The action "Guess value" works like the converter Filename-Tag - only that you can determine how the source is composed, you are not limited to the filename.
So you enter a generating format string as source.
The target is defined as matching format string in which you match the pattern in the text with fields.
In your case you take the contents of the field ARTIST and then you assign parts of the resulting string to the DUMMY field and the part in parenthesis to ARTIST.
See also the documentation on format strings:
Matching format strings are used for simple pattern matching of existing strings, e.g., file names to import tags from or lines in a text file that describe tag contents to be imported.
Generating format strings are used to create new strings, e.g., file names when renaming, field contents when formatting tag fields, data when exporting, but also file names for playlists or cover images that are parameterized with tag contents. Generating format strings can be combined with scripting functions.
BTW Your current Artist entries all seem to include the song title as well. Do these files already have an accurate %title% field stored? This can be easily filled in at the same time by making the below tweak to my earlier suggestion, replacing the %dummy% placeholder with %title%.