Jmeter - sends requests fine using parameter but not using body - jmeter

Question

I am load testing my website API. i try and send a request to the website for login which requires username and password. I initially set up http header manager, cookie manager and http request.
I enter name(username,password) and value parameter(say vis,new respectively)in parameters, then hit start and the tests completes successfully. On the other hand, if i type the key value pairs as dictionary {"username":"vis","password":"new"} in body tab and then run again, then request header has the post data(as seen from view results tree) but my server is not recognizing the post data. It just returns error as if no post data was sent.
I also made the Content-Type as application/json for the body tab method but i still get the same error.
Some error screenprints:
Header setting
Request parameters
View results in tree - request header:
Connection: keep-alive
Content-Type: application/json
Content-Length: 35
Host: url.com
User-Agent: Apache-HttpClient/4.5.6 (Java/1.8.0_191)
request-body:
POST https://url.com/accounts/login.json
POST data:
{"username":"vis","password":"new"}
[no cookies]
So, my request works when i send it under parameters tab but not while using body data tab.

Answer 1

You're not able to login because your application doesn't support the way you're providing the credentials.
If Python request works fine for your - just record it using JMeter's HTTP(S) Test Script Recorder
Prepare JMeter for recording. The fastest way is using JMeter Templates feature.
From JMeter's main menu choose File -> Templates -> Recording and click "Create"
Open HTTP(S) Test Script Recorder and click "Start"
Amend your Python script to use JMeter as the proxy:
import requests
jmeter = {
'http': 'http://localhost:8888',
'https' : 'http://localhost:8888'
}
requests.post("https://url.com/accounts/login.json",data={"username":"vis","password":"new"}, proxies=jmeter)
Execute your Python script
JMeter will capture it under the Recording Controller. This would be the request you should be using for your testing: