HTTP POST sending empty [object Object] and Undefined Values

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6 
    client.println(postData.length() + "\r\n\r\n");

client.println();

You're sending 4 CRLF's, there should be only two:

    client.println(postData.length());
    client.println();

client.println(postData);

Why the line ending? You're sending more data than you specified in the Content-Length header.

Your data seems to be JSON, so why do you tell the server to interpret it as plain text?

On top of that:

RFC7230§6.1:
A client that does not support persistent connections MUST send the "close" connection option in every request message.

Read the response from the server to see if it gives an error message.