HttpClient PostAsync() never return response

Since you are using .Result or .Wait or await this will end up causing a deadlock in your code.

you can use ConfigureAwait(false) in async methods for preventing deadlock

like this:

string responseContent = await response.Content.ReadAsStringAsync().ConfigureAwait(false);

you can use ConfigureAwait(false) wherever possible for Don't Block Async Code .

Since you are using .Result, this will end up causing a deadlock in your code. The reason this is working in a console application is because console applications don't have contexts, but ASP.NET apps do (see Stephen Cleary's Don't Block on Async Code). You should make the Signin method in your controller async and await the call to _authenticationService.Authenticate to resolve the deadlock issue.