have you Splunk Light Free or Splunk Light?
If you have Splunk Light Free, Alerting isn't an available feature.
If you have Splunk Light, you can run an alert and send an event to Splunk Enterprise in many ways: via syslog, event in in a forwarded index or running a script.
to use syslogs, you can follow this documentation:
Otherwise, you should configure something like an alert action that sends events across via HTTP Event Collector using the TA-SendtoHEC App ( https://splunkbase.splunk.com/app/3508/ ) and enablig HEC on Splunk Enterprise (for more infos see https://docs.splunk.com/Documentation/Splunk/8.0.1/Data/UsetheHTTPEventCollector ).
But maybe the easyest way is to send alerts to a mailbox and monitor this mailbox using the Splunk for IMAP App ( https://splunkbase.splunk.com/app/27/ ).