Add ticket thread from my application

Hi,

Currently, I have an application that is consuming Zoho Desk API, now I am trying to add threads to tickets from a FORM in my site, I dont get how can I do it using POST /api/v1/tickets/{ticket_id}/sendReply...

I have the ticket id that I would like to add the thread, but I don't know which CHANNEL should I use, I tried to use WEB, but when I send like this:

{
  "channel" : "WEB",
  "content" : "Test"

}

the return is an error 500:

{
    "errorCode": "INTERNAL_SERVER_ERROR",
    "message": "An internal server error occurred while performing this operation."
}

Should I send any other parameters to make this work? The API documentation does not mention anything about this...

Thanks in advance.