Create a new field based on source and apply to all the events from that source at index time

I have Splunk looking at specific directories for our Geb Unit test reports where reports for each browsers (Chrome, IE, etc) are in their own folders. Test reports are in XML files and each events are based on testcase tag: I would like to be able to create a new field called ***browser_type*** based on what test folder the reports came from, i.e. if the report came from IE test folder, then ***browser_type=IE*** and add that field to all events of all Geb test reports that will be read from that folder. Any idea how to go about this?