You need a props entry for the sourcetype WinEventLog:Security, that includes a FEW REPORT- settings in it. Make sure these come after those already shipped with Splunk, so that the Message field actually exists. You can check for that using splunk btool props list WinEventLog:Security , and see which REPORT- or EXTRACT- creates tue Message field.
After that, create some transforms stanzas according to what you mentioned in props.conf, and simply put in the regex from above, SOURCE_KEY is Message, and you should be pretty good to go.
There is a bunch of documentation on all of those topics available, so I think you should make it from here 😉
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