This is already discussed here : https://answers.splunk.com/answers/233581/how-to-send-data-from-ibm-as400-to-splunk-via-sysl.html
You can use syslog for sending the data and make sure you use this app for field extraction once data is received : https://splunkbase.splunk.com/app/633/#/details
I have checked that link , I don't think anybody have accepted that answer .
Also do you have any documentation to use that AS400 i series TA?
yeah.. true. AS400 is pretty old, you can ask your IBM support for documentation on setting up syslog, they should have the official one, I could point you to some links but not sure how reliable they would be. All you have to do is get that syslog up and running 🙂