sa is a system administrator account which comes with sql serverSQL Server by default. As you know might already know, you can use two ways to log in to SQL Server.
Therefore there are connection strings which suitable for each scenario (such as windowsWindows authentication, localdb, etc.). Use https://msdn.microsoft.com/en-us/library/jj653752(v=vs.110).aspx#sqlserverSQL Server Connection Strings for ASP.NET Web Applications to build your connection string. These are XML tags. You just need a value of connectionString.
