My apologies for opening topic.
Problem was that my development setup point to http not https instance of discource.
When I switched to https endpoint it start working.
I get confused because some of api endpoints woking over http but this on does not.
For future readers here how i logout users by C#
var url = $"/admin/users/{user.user.id}/log_out";
using var formData = new MultipartFormDataContent{
{ new StringContent(_forumConfigModel.ApiKey), “api_key” },
{ new StringContent(_forumConfigModel.User), “api_username” }
};
var response = await _httpClient.PostAsync(url, formData);