A quick web search does not show great documentation for Fronius push service, but I did find some things. I would use http. Use api.thingspeak.com as the server and use the url for address from the write data API. something like '/update.json.
But from the doc and from what you have shown, I cant quite see how to specify the post body. From the above, it looks like the device generates an xml, but you will need to make sure its parameters match the ThingSpeak api endpoint. Or you might be able to specify the data as part of the address if it accepts variable input similar to the upload field shown above.
My advice would be to buy a cheap microcontroller (like esp32 or Arduinio) and use that as a proxy between the two.
