Run your search as you have it, pipe your results to erex (may favorite hidden gem in Splunk).
The first "SamLogon" value is what you name the field, it's arbitrary. The second is the value that Splunk searches for.
Run the search, the examine the Job Inpspector, it will provide the regex that Splunk used to find the value.
You can fine tune it from there, but it is a fantastic starting point...