The ON command is passed as body of the POST http request, it is not part of the URL.
here is an generic example of a http post action with Tasker: https://www.triggercmd.com/forum/topic/35/trigger-commands-with-a-tasker-http-post-action
In your case here are the variables you need to set
Server:Port: https://openhabianpi.8080
Path: rest/items/BW4
Data/File: ON