Hi,
By default Splunk use their own signed certificate with management port (8089), so to send request in secure mode you can configure Splunk management port with self-signed certificate or third party certificate, if you do not want to configure self-signed certificate or third party certificate then import Splunk CA from $SPLUNK_HOME/etc/auth/ to your browser if you are using POSTMAN Chome App, have a look at Postman Docs http://blog.getpostman.com/2014/01/28/using-self-signed-certificates-with-postman/
If you want to configure self-signed certificate in this case as well you need to import root CA in your browser Trusted Certificate store.
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