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"}}'