Is your connection string something like this:
Persist Security Info=False;User ID=DOMAIN\USER;Password=PASSWORD;Initial Catalog=AdventureWorks;Server=MySqlServer
That is not going to work. This is what you would use for SQL authentication.
To use Windows authentication, this needs to be added to the connect string:
Integrated Security=SSPI
But you cannot specify the user and password. It uses the security of the login that started the application.
Plus, using a service account to run an app is probably not a good idea. Create a SQL login, grant it the minimum level of rights, and use that to connect.