NOTE:
I cannot state with certainty that the attachment will work because we have no idea how your instance of Splunk is implemented.
Assuming you have the necessary SPLUNK jars in LISA_HOME/hotdeploy, try developing a JSR-223 /Javascript step to make the Splunk query call. Alternatively, if you have custom JARs, you could invoke your logic by calling the exposed API.
Regardless, you will need to work with your Splunk team to get the proper connection parameters, login IDs, query syntax, contexts, etc. And, you will need to determine how to parse the query response output.
The attached file might help you get started.