In order for Splunk to receive data from the ElastiFlow Unified Flow Collector you must first configure a Data Input & an Index.
There are 5 steps to set up a Data Input & Index
Create a Data Input: Settings -> Data Inputs -> HTTP Event Collector -> +Add New
Give it a Name, click Next
Source Type -> Select -> Select Source Type -> Log To Metrics -> log2metrics_keyvalue
Select Allowed Indexes (pick the ElastiFlow Index you want to use, if one does not exist click "Create a new index")
Verify these sections and click Review:
Click Submit
Copy this Token Value and use it in your ElastiFlow configuration here