How to use the Open Trigger in workflows? What is the API endpoint?

How to use the Open Trigger in workflows? What is the API endpoint?

When designing a workflow, I want to use the Open Trigger ("ON USING API")



It allows to generate an API key, but where is this API key to be used? What's the endpoint?



I found this endpoint "Trigger Contacts" in the API docs, https://www.zoho.com/campaigns/help/developers/trigger-contacts.html

But it does not mention anywhere the above API key. The endpoint needs 
  1. a "custom workflowkey" https://campaigns.zoho.com/api/v1.1/workflow/[custom workflowkey]/action/trigger?details={key-value pairs}
  2. a "triggerkey": is that the API key? Then why naming it a triggerkey, not "apikey"
  3. a "listkey": why do we need a listkey? What if our audience is not broken down into lists?
Would love if you could explain in details exactly how to use this endpoint, thanks!