CreateRoute.cs

// https://documentation.mailgun.com/docs/mailgun/api-reference/openapi-final/tag/Routes/#tag/Routes/operation/post-v3-routes
// This sample works with .net framework
private static async Task<Route> CreateRoute(string recipientAddress, string forwardAddress)
{
    RestClientOptions options = new RestClientOptions(new Uri("https://api.mailgun.net/v3"));
    options.Authenticator = new HttpBasicAuthenticator("api", ApiKey);

    using (RestClient client = new RestClient(options))
    {
        //Url encoded format, example: https://example.com/api?priority=0&description=it%27s+a+new+route&expression=match_recipient%28%22user%40example.com%22%29&action=%5B%27forward%28%22forward%40example.com%22%29%27%5D

        RestRequest request = new RestRequest();
        request.Resource = "/routes";
        request.AddParameter("priority", "0");
        request.AddParameter("description", "it's a new route");
        request.AddParameter("expression", $"match_recipient(\"{recipientAddress}\")");
        request.AddParameter("action", $"forward(\"{forwardAddress}\")");

        
        var response = await client.ExecutePostAsync(request);

        // Parse the JSON and select only the "route" part
        JObject jsonObject = JObject.Parse(response.Content);
        Route route = jsonObject["route"].ToObject<Route>();

        return route;
    }
}