No route found for contact_message

From Chrome Postman call a POST request over json.

URL: contact_message?_format=json
Basic Auth...
Content-type: application/json

Body:

{
    "contact_form":[{"target_id":"my_contact_form_machinename"}],
    "name":[{"value":"my_login_name"}],
    "mail":[{"value":"my_email"}],
    "subject":[{"value":"REST contact form"}],
    "message":[{"value":"REST message body"}]
}

Message:
{"message":"No route found for \u0022POST \/contact_message\u0022"}

Maybe there is a solution?