Point connectionstring in dbml to app.config

Can I just point the connection string in Dbml.designer.cs to the connectionstring in the app.conf? I wrote the code below which it successfully point to the app.config.

public leDataContext() : 
    base(ConfigurationManager.ConnectionStrings["leConnString"].ToString(), mappingSource)
    {
        OnCreated();
    }

However whenever i modify or add a table into the dbml, it will start to auto replace that code into this

 public leDataContext() : 
            base("Data Source=.\\SQLEXPRESS;AttachDbFilename=\"D:\\My Projects\\App_Data\\le.mdf\";Integrated Security=True;Connect Timeout=30;User Instance=True", mappingSource)
    {
        OnCreated();
    }

I have expanded the "Connection" option. Set "Application Settings" to False