You can find the HTTP Event Collector Token in Splunk, go to Splunk Settings > Data inputs > HTTP Event Collector.
See more details at HTTP Event Collector walk through
To verify the token via curl command
hec_host=<splunk-host> hec_port=8088 hec_token=<splunk-token> curl -k "https://${hec_host}:${hec_port}/services/collector/event" -H "Authorization: Splunk ${hec_token}" -d \ '{"host":"test-host","index":"jenkins_console","sourcetype":"json:jenkins","source":"logger://dummy","event":{"level":"INFO","log_source":"cmdline","message":"Test HEC"}}'