using System;

using System.Web;

public class Program

{

    public static void Main()

    {

        var uri = new Uri("https://P1.P2.com/ProjectName/_build?definitionId=123");

        string subdomain = uri.Host.Split('.')[0];

        string projectName = uri.Segments[1];

        var query = HttpUtility.ParseQueryString(uri.Query);

        string definitionId = query["definitionId"];

        var builder = new UriBuilder()

        {

            Scheme = "https",

            Host = "dev.azure.com",

            Path = $"{subdomain}/{projectName}_apis/pipelines/{definitionId}/runs",

            Query = "api-version=6.0-preview.1",

        };

        Console.WriteLine(builder.ToString());

    }

}