The regular method of field extraction (using IFX utility OR from Settings-> Fields -> Field extractions) doesn't allow you to extract the fields from another fields, unless you can write a regex off the _raw fields that will extract the value that you need. You would need to use Field Transforms to use another fields (which should be available before the fields transform is run, it can't include auto extracted fields. See this for more information on order of search time field extractions). So, first you need to create a Field transform (Settings-> Fields -> Fields transforms, select SOURCE_KEY as your original field name) and then create a Field extraction which refers to that transform (Settings-> Fields -> Field extractions , type should be 'Uses transform' and provide name of transform).
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