REST API response with 400 Bad request when Content-length header is givin in POST - api

Question

I have a Website and a python flask RESTFUL API. When testing the API with Postman it works just fine, but the same request using XMLHttprequest in js gives me a 400 BAD Request code. I looked at the headers send and replicated the XMLHttprequest headers in Postman, to get a 400 BAD REQUEST Code. If i remove the Content-Length header from the request, it works fine. My Postman code (HTML, without content-length):
POST /user/181453766040485888 HTTP/1.1
Host: 127.0.0.1:5000
Content-Type: multipart/form-data; boundary=---
-WebKitFormBoundary7MA4YWxkTrZu0gW
Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Connection: keeep-alive
Host: 127.0.0.1:5000
Origin: http://localhost:52014
Referer: http://localhost:52014/settings.php
User-Agent: Mozilla/5.0 (Windows NT 10.0;
Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36 OPR/57.0.3098.106 (Edition Campaign 76)
cache-control: no-cache
Postman-Token: 5dd34b0b-965c-4d31-83b9-2364b03c0aa2
Content-Disposition: form-data; name="status"
0
Content-Disposition: form-data; name="username"
0
Content-Disposition: form-data; name="token"
0
Content-Disposition: form-data; name="display_names"
1
Content-Disposition: form-data; name="messages"
1
Content-Disposition: form-data; name="roles"
1
Content-Disposition: form-data; name="votes"
1
------WebKitFormBoundary7MA4YWxkTrZu0gW--
My js function (for testing):
function save() {
var formData = new FormData();
formData.append("status", 0);
formData.append("username", 0);
formData.append("token", 0);
formData.append("display_names", 0);
formData.append("messages", 0);
formData.append("roles", 0);
formData.append("votes", 0);
const Http = new XMLHttpRequest();
const url='http://127.0.0.1:5000/user/181453766040485888';
Http.open("POST", url);
Http.onreadystatechange=(e)=>{
console.log(Http.responseText)
}
Http.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
Http.send(formData);
}
my API function that handles the request (abstract):
def post(self, id):
if database.execute(database.SAVE_USER_SETTINGS.format(id, request.form["status"], request.form["username"],
request.form["token"], request.form["display_names"],
request.form["messages"], request.form["roles"],
request.form["votes"])):
return (id, request.form["status"], request.form["username"],
request.form["token"], request.form["display_names"],
request.form["messages"], request.form["roles"],
request.form["votes"]), 201
return {}, 500