https://docs.splunk.com/Documentation/Splunk/8.2.2/Data/AboutHECIDXAck
- Section: About channels and sending data
Sending events to HEC with indexer acknowledgment active is similar to sending them with the setting off. There is one crucial difference: when you have indexer acknowledgment turned on, you must specify a channel when you send events. The concept of a channel was introduced in HEC primarily to prevent a fast client from impeding the performance of a slow client. When you assign one channel per client, because channels are treated equally on Splunk Enterprise, one client can't affect another. You must include a matching channel identifier both when sending data to HEC in an HTTP request and when requesting acknowledgment that events contained in the request have been indexed. If you don't, you will receive the error message, "Data channel is missing." Each request that includes a token for which indexer acknowledgment has been enabled must include a channel identifier, as shown in the following example cURL statement, where <data> represents the event data portion of the request

Explanation/Reference: http://docshare02.docshare.tips/files/22651/226514302.pdf

because transforms.conf is the right configuration file to state the regex expression. https://docs.splunk.com/Documentation/Splunk/8.1.0/Admin/Transformsconf Reference:
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