Trying to send post requests to a parent page from JS by calling

function sendPostMessage(message) {
    console.log("sending post");
    var post = { conf: message };
    parent.postMessage(post, "*");
}

from c# as in

private void post_to_parent(string message)
{
    Debug.WriteLine($"Sending post: {message}");
    ScriptManager.RegisterStartupScript(this, this.GetType(), Guid.NewGuid().ToString(), $"sendPostMessage("{message}");", true);

}

But I get no response.

If I call the function from the same JS everything works fine.
If I call it from the C# function, I get the message in the debug console, but nothing in the JavaScript console. No message, nothing.

I am able to call other JS functions the same way and it works fine.

I am on an ASP.NET Web Form.