The correct answer is to use the "HTTP extensions for On" field and the right combination of carriage returns and line feeds.
Many thanks to
https://github.com/menecio to help me find this out!
For below example this becomes:
Virtual Output Command - HTTP extensions for On; Authorization: Bearer 6fd2c9f940c0f3243c3d1b3d1bb9e76\r\nContent-Type: application/json
Virtual Output Command - HTTP Post command for ON; {"mode":"sync", "messageType":"0f8453105c3b9faf7886", "messages":[{"celcius":\1, "humidity":66.6, "timestamp":"2017-01-15T11:55:37.930Z"}]}
Note that the \n is added by using Enter key in the field box so if you use Enter all you have to add is the "\r". You can see the final string by hoovering over the field content.
Now the next step; passing on values from the Loxone system.