Beginner question: How to get the authorizationcode programmatically?

I'm trying to authorize myself to use the strava API. I managed to get an access token and specifying the scopes. In order to be able to write data to one of my activities I need to pass in an authorization code (with the great help of ​@Jan_Mantau).  On the internet I found examples of getting the url that would display the code.  

static async Task Main(string[] args)
{
    // Step 1: Send user to authorization page
    var queryParams = new Dictionary<string, string>
    {
        { "client_id", clientId },
        { "response_type", "code" },
        { "redirect_uri", redirectUri },
        { "scope", scope }
    };
    var authorizeUri = $"{authorizeUrl}?{string.Join("&", queryParams.Select(kvp => $"{kvp.Key}={Uri.EscapeDataString(kvp.Value)}"))}";
    Console.WriteLine($"Open the authorization url: {authorizeUri}");

    // Step 2: User authorizes on page and get redirected to the redirect)uri with a code
    ...

}

 https://www.strava.com/oauth/authorize?client_id=99999999&response_type=code&redirect_uri=http%3A%2F%2Flocalhost%2Ftoken_exchange&scope=activity%3Aread_all%2Cactivity%3Awrite

using this url in a browser, gives me the authorization page on which I click approve after which I get redirected to the localhost url specified.  In that localhost url the code that I need is displayed in the querystring….  

In my use case I want to receive the authorization code in my code so I can use it to do the actual write on the activity.   I know I have to authorize it one time manually. 
How can I get the authorization code in code  by calling the authorizeUri?   For development purposes I'm writing code that runs on a laptop. In a later stage I'll turn my code into a mobile app.