Again, when you see "\"" in the REPL/iex then the string contains only a single double quote. Of course when you copy the inspected value and paste it as is into some other program that interpretes input as is, then it will go wrong, as both programs work with different levels of abstractions.
Thats why I’m asking how exactly you are testing that the receiver sees the actual backslash.
An HTTPoison.post!("http://example.com", "{\"year\":\"2020\",\"p\":\"1002\"}") will issue an HTTP post request with the following body:
{"year":"2020","p":"1002"}
You can verify using wireshark or tcpdump or any other network sniffer if you want (and are not using HTTPS).
Also I assume, that when the other side returns a 400, then it usually tells you in the body or the headers what you did wrong.
Therefore again, please show us the code how you send the request via HTTPoison and how you do via postman. Please include what headers you send from the postman side.
