Request api on behalf of end user - cannot make comment as internal

Hello,

I am currently working on a use case where I use the requests api to create a ticket on behalf of a customer.

This works fine but i'd like to be able to have them add comments from the requests api as internal. I can't seem to get this to work. The comment will add as public no matter what i do it seems, what am i doing wrong? is it even possible to add a comment on behalf of an end user, as internal?

(python )

body = {
    "request": {
        "comment": {
            "public": False, ( i have tried "is_public" as well)
            "body": "new ticket comment."
            
        }
    }
}

thanks