How to write logic based off of selection in Picker

Question

I have an application that based off of your zipcode gathers weather information from the OpenWeather API. I am trying to change it to have a drop down that allows you to select the city that you want information for instead of inputting the zipcode, but I am not sure how the logic works out.

XAML

<StackLayout>
        <!-- Screen Controls and Text -->
        <Label Text="Welcome to Isaac's Weather App!:"
               HorizontalOptions="Center"
               HorizontalTextAlignment="Center"
               VerticalOptions="CenterAndExpand"
               FontSize="30"/>
        <Picker x:Name="picker"
                Title="Select City"
                SelectedItem=""/>
        <Button x:Name="ShowTemp"
                VerticalOptions="CenterAndExpand"
                Padding="30"
                Text="Show Current Temp"
                Margin="10"
                Clicked="ShowTemp_Clicked"/>
    </StackLayout>

API Code

 public MainPage()
        {
            InitializeComponent();

            var citylist = new List<string>();
            citylist.Add("LA");
            citylist.Add("New York");
            citylist.Add("Orlando");

            picker.ItemsSource = citylist;

        }

        private void ShowTemp_Clicked(object sender, EventArgs e)
        {
            using (WebClient wc = new WebClient()) //New webclient
            {
                wc.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded"; //which web client

                try
                {
                    string jsontext = wc.DownloadString($"https://api.openweathermap.org/data/2.5/weather?zip={EntryZipCode.Text}&appid={APIKEY}%units=imperial"); //Get json text
                    JObject jo = JObject.Parse(jsontext); //Parse json
                    JObject main = JObject.Parse(jo["main"].ToString()); //parse main section
                    WeatherGV.CurTemp = main["temp"].ToString(); //get temperature
                    WeatherGV.CityName = jo["name"].ToString(); //get city name
                    WeatherGV.LowTemp = jo["temp_min"].ToString(); //get low temp
                    WeatherGV.HighTemp = jo["temp_max"].ToString(); //get high temp
                    WeatherGV.WindSpeed = jo["speed"].ToString(); //get wind speed
                    WeatherGV.WindDirection = jo["deg"].ToString(); //get wind direction
                    WeatherGV.Pressure = jo["pressure"].ToString(); //get air pressure
                    WeatherGV.Humidity = jo["humidity"].ToString(); //get humidity
                    WeatherGV.Sunrise = jo["sunrise"].ToString(); //get sunrise
                    WeatherGV.Sunset = jo["sunset"].ToString(); //get sunset
                    Navigation.PushAsync(new WeatherPage());
                }

I know I need to convert the city into my EntryZipCode variable that I had from before, but I don't know how to get the city into the zipcode.