Send ENV variable with the POST API call

[samsonw]

Thanks a lot for creating this plugin, it's very useful.

Currently, every API call seems to have pre-defined all the fields that will be sent, it will be better if we can send the system ENV variable along with the payload?

Basically, we're seeking a way to send some custom data in the payload. System ENV is a workaround for us right now. If there is a way to define customized fields, that will be very useful.

Thanks for the help!