Parse GeoJSON text attribute to geometry and test intersection with another WKT attribute

I have some features that contain an attribute with a GeoJSON string, I also have a Multipolygon in WKT passed as a global parameter to the workspace, what I have to do is to parse each geometry from the attributes (in geoJSON) merge them into a multipolygon and test if this intersects at least once the geoJSON mulitpolygon. At the moment I cannot parse the GEOJSON from the attributes, I tried to use geometryExtractor but the transformer instead of extracting the GEOJSON string extracts the geometry of the incoming feature (which is unrelated), i tried the feature reader but I get an error because the reader is looking fro a file path while I submit an attribute. I parsed the geoJSON with the validation online and it id fine. How can I accomplish this?