You just need to genereate a GUID.
you can use any 'guid generator' tool to do so - an online version like this will do https://www.guidgenerator.com then you can just use that guid in your POST/GET:
curl https://http-inputs-<customer>.splunkcloud.com/services/collector/raw -H "X-Splunk-Request-Channel: FE0ECFAD-13D5-401B-847D-77833BD77131" -H "Authorization: Splunk BD274822-96AA-4DA6-90EC-18940FB2414C" -d '<raw data string>' -v
Thanks for this! The documentation isn't clear that this can be ANY guid, and isn't something generated by Splunk. Further, do note that this must be a true GUID - you cannot use any arbitrary string or number.