I would use the GeometryReplacer transformer and the WKT (Well Known Text) format.
You will need to format the geometry attribute to match the WKT (add the POLYGON definition).
It should look like this: POLYGON ((0 0,10 0,10 10,0 10,0 0))
That is POLYGON ((X Y,X Y,X Y,X Y,X Y)), using the comma to separate vertices and a space to separate X and Y).
Hope this helps.
@kat The GeometryReplacer that @erik_jan mentions has any options for different supported geometry encodings. If you know what encoding then the GeometryReplacer might work 'out-of-the-box'. Your data looks like JSON, but not GeoJSON which looks more like:
{"type":"Polygon","coordinates":[[[-123.3,49.3],[-123.3,49.1],[-122.3,49.1],[-122.3,49.3],[-123.3,49.3]]]}
So I think if you replace your () (Parentheses) in your geometry with [] (braces) and then add the wrapper:
{"type":"Polygon","coordinates": ... }
you can use the GeoJSON encoding in the GeometryReplacer.
Also seems to be additional leading and trailing [] which need to be dropped for GeoJSON.
Example workspace (2019.1): geojson.fmw