System.ArgumentNullException: Value cannot be null. (Parameter Password)

I am trying to connect to sql db using username and password. I have saved the password in the windows environment and named it "HSPA_DBPassword" and here is what i have:

appsettings.json

"ConnectionStrings": {
      "Default": "server=rwahdan; database=hspa; User Id=sa;"
},

program.cs:

       var builder2 = new SqlConnectionStringBuilder(
       builder.Configuration.GetConnectionString("Default"));
    
       builder2.Password = builder.Configuration.GetSection("DBPassword").Value;
        

        var ConnectionString = builder2.ConnectionString;

        builder.Services.AddDbContext<DataContext>(options =>
            options.UseSqlServer(ConnectionString, option =>
            {
                option.EnableRetryOnFailure(5, TimeSpan.FromSeconds(10), null);
            })
        );

there is mistake that I have builder and builder2 and in my cs file i have builder set to handle the configuration and I am not sure why is that? I don't have a startup file as well.

with my code I am getting error that the password is null but it is saved in windoes invironemt and the appsettings.json i removed the password.

error:

Unhandled exception. System.ArgumentNullException: Value cannot be null. (Parameter 'Password')