In transforms.conf, we're telling it to match as many non-space characters as possible at the beginning of a line. Then, assign it to the hosts value. In props.conf, note that you need to use TRANSFORMS-xxx instead of REPORT-xxx, since this is happening at index time.
Props.conf is what tells Splunk to actually apply the transform we defined to your data.
I'm assuming from your current props.conf that ErrorMissingObject is the sourcetype for these entries.
You can just add the TRANSFORMS-host line to what's already there.