To understand this, consider the following code:

        //Dummy DataTable
        DataTable dt = new DataTable();
        dt.Columns.Add("EmployeeName");
        dt.Columns.Add("EmployeeId");
        for (int i = 1; i <= 10; i++)
        {
            DataRow dr = dt.NewRow();
            dr[0] = "Name" + i.ToString();
            dr[1] = i;
            dt.Rows.Add(dr);
        }

        //Configure DropDownList
        DropDownList1.DataTextField = "EmployeeName";
        DropDownList1.DataValueField = "EmployeeId";
        DropDownList1.DataSource = dt;
        DropDownList1.DataBind();

        //default value
        string defaultText = "Name5";

Approach 1:

        foreach (ListItem item in DropDownList1.Items)
        {
            if (item.Text == defaultText)
            {
                item.Selected = true;
                break;
            }
        }

Approach 2:

        DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByText(defaultText));

Happy Coding!!