Now, in 2024, 7 years later it is still not very clear how one applies a saved extraction regex to an existing search to extract fields from the search. Especially without access to the various server side configuration files. Splunk has grown long in the tooth, dementia encroaching.
Reality: You probably can't do it simply.
If you have a sourcetype X. The extractors you saved will only run against the base, plain data set sent as X, not against your search, and they run against the base sourcetype automatically. If it was going to work, it would already be working and you would already have your field.
Now, if your search does any kind of transformations like for example pulling log fields out of JSON data using spath, messing around with _raw or similar, the extractor you created isn't going to run against that resulting data set. I know, I've tried. The extractors get applied before that part of the search runs.