Get AuthToken for SharepointOnline in dotnet core - sharepoint

Question

I'm able to get an auth token for Sharepoint in Postman using the following parameters:
URL : https://accounts.accesscontrol.windows.net/{tenantId}/tokens/OAuth/2
Method: POST
Headers :
- Content-type: application/json
Body :
Resource: 00000003-0000-0ff1-ce00-000000000000/{myorganization}.sharepoint.com#{tenantId}
grant_type: client_credentials
client_id: {appId}#{tenantId}
client_secret: {myclientsecret}
Now, I'm trying to reproduce this POST http call in dotnet core, but I'm getting always a 400 (Bad request) response..
Here is my code :
var body = new AuthBodySPO
{
Resource = resource,
client_id = appId,
client_secret = appSecret,
grant_type = "client_credentials"
};
using (var client = new HttpClient())
using (var request = new HttpRequestMessage(HttpMethod.Post, url))
{
var json = JsonConvert.SerializeObject(body);
using (var stringContent = new StringContent(json, Encoding.UTF8, "application/json"))
{
request.Content = stringContent;
using (var response = await client
.SendAsync(request, HttpCompletionOption.ResponseHeadersRead)
.ConfigureAwait(false))
{
response.EnsureSuccessStatusCode();
}
}
}
Anyone has an idea ?
Thanks