The key is auto-generated by default, but you can override that behavior by specifying your own key. You have to perform an eval to concatenate the two fields together to make a single key. The key will not appear in an inputlookup of the KV store as long as you specify the key_field argument in your outputlookup.
| eval key=IncidentID.CompanyName
| outputlookup append=true key_field=key
See also:
http://dev.splunk.com/view/webframework-developapps/SP-CAAAEZH
http://docs.splunk.com/Documentation/Splunk/latest/SearchReference/outputlookup
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