How to correctly pass body to spring rest template? - spring

Question

SOLUTION: The API endpoint to get this specific information is the only one different from all the other ones. Thus I was trying to contact an outdated one.
Everything described in the post works like a charm.
Using kotlin and spring, I want to send a POST request to an API.
This is a curl, generated via postman, that works correctly:
curl --location --request POST 'URL' \
--header 'Content-Type: application/json' \
--data-raw '{
"username":"username",
"password":"password"
}'
However, with spring + rest template using the following code, it seems that the body is not parsed correctly, as the API I am contacting sends me back a 401, that occurs when password/username provided are incorrect/missing.
val body = JwtQuery(getUsername(), getPassword())
return restTemplate.postForObject(url, body, Jwt::class.java)!!
This is the JwtQuery class:
data class JwtQuery (
val username: String,
val password: String
)
Note that getUsername() and getPassword() return the expected value.
What am I doing wrong ?
Edit 1: setting headers
Using:
val body = JwtQuery(getUsername(), getPassword())
val headers = HttpHeaders()
headers.contentType = MediaType.APPLICATION_JSON
val bodyEntity = HttpEntity(body, headers)
return restTemplate.postForObject(url, bodyEntity, Jwt::class.java)!!
Still returns the same error (401)
edit 2: marshaled JwtQuery
val body = JwtQuery(mesProperties.getUsername(), mesProperties.getPassword())
val marshaled = ObjectMapper().writeValueAsString(body)
println(marshaled)
Output:
{"username":"username","password":"password"}