How to validate URL while calling an API in C# ?

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C#

.NET

API

UWP

I'm very new to consuming APIs. I'm calling a weather API in C#.Currently I'm making weather Application in UWP ( Universal Windows Platform ). When I run the following code .

var client = new HttpClient();
                HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Get,
                    "api.openweathermap.org/data/2.5/weather?q=" + cityName + "," + countryName);
                var response = await client.SendAsync(request);
                if (response.StatusCode == System.Net.HttpStatusCode.OK)
                {
                    var result = response.Content;
                    string restultString = await result.ReadAsStringAsync();
                    // var data = JsonConvert.
                    var data = JsonConvert.DeserializeObject<RootObject>(restultString);
                    lblWeatherResults.Text = data.sys.country.ToString() +" - "+ data.main.temp.ToString();
                }

I receive error which is asking me to provide base url and or absolute URL. I'm unable to figure out why this error coming.
this is the error i'm getting .

An Invalid request URL was provided. The ruquest URL must either be an absolute URL or BaseAddress must be set. 

here is the API which i'm trying to configure into my application using C#.

What I have tried:

I search it on Google but could't find useful material. It is asking for the correct URL but URL Format specified here is exactly the same which i'm trying .