Hi @PAUL
There are multiple components involved and this needs to be troubleshot from individual components.
Have you verified if you are able to see the event message "This is a demo" at the event hub? We want to make sure if the issue is between the function app to the event hub or the event hub to ELK.
You can use service bus explorer to view the events from the event hub. If you need any help in verifying this please let me know so we can connect offline.
Please send an email with the subject line “Attn: Mayank” to AzCommunity[at]Microsoft[dot]com referencing this thread along with subscription ID, function app, and event hub details.