public override bool TryAuthenticate(IServiceBase authService, string userName, string password)
{
    if (userName == "admin" && password == "test")
        return true;

    return false;
}

public override IHttpResult OnAuthenticated(IServiceBase authService, IAuthSession session, IAuthTokens tokens, Dictionary<string, string> authInfo)
{
    try
    {
        base.OnAuthenticated(authService, session, tokens, authInfo);

        // Save the browser cookie.
        if (authService.Request is IHttpResponse httpRes)
            httpRes.Cookies.AddPermanentCookie(HttpHeaders.XUserAuthId, session.UserAuthId);

        // Save the user session object (ServiceStack stores it in the in-memory cache).
        authService.SaveSession(session, this.SessionExpiry);

        return new HttpResult(HttpStatusCode.Accepted);
    }
    catch (Exception ex)
    {
        return new HttpResult(HttpStatusCode.ExpectationFailed);
    }
}

{
    "UserName": "admin",
    "Password": "test",
    "RememberMe": true }
}

using (var client = new JsonServiceClient("http://localhost:24131"))
{
    client.AddHeader("Username", "admin");
    client.AddHeader("Password", "test");
    client.Post<HttpWebResponse>("/auth/credentials?format=json");

    response = client.Get(new GetProducts());
}