let me understand what's uour problem: yo uaven't any result or you have only "error_name=ErrorMsg:"?
probably there's an error in field extraction caused by the space in the string.
Splunk automatically extract fields when finds field=value, but if there's a space thinks that the value is only the first part of the string.
If you share a sample of your log, I could help you in the regex creating.
if this is your full event log, please try this regex:
| rex "error_name\=(?<error_name>.*)"
that you can test at https://regex101.com/r/b0ZBaA/1
Obviously, if your event is different (e.g. something other at the end), this regex isn't still correct (for this reason I asked a sample of your log!).