Custom Source Type - Group by Column in TSV file

I have a TSV file im uploading into Splunk, I'd like to be able to group by a column in the file itself. So far I'm using the Splunk Source Type UI to create the custom source type, used the field tab to label the columns and I'm missing the step to group by a column. Can anyone please advise? Screen shot shows the setup below, I'd like to group by Session ![alt text][1] [1]: /storage/temp/284687-screen-shot-2020-03-09-at-91811-am.png