Hi,

Please refer below code.

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        List<string> countries = new List<string>();
        DataTable dtCountries = new DataTable();
        dtCountries.Columns.Add("CountryId");
        dtCountries.Columns.Add("CountryName");
        dtCountries.Rows.Add(1, "India");
        dtCountries.Rows.Add(2, "Afganistan");
        dtCountries.Rows.Add(3, "Pakistan");
        dtCountries.Rows.Add(4, "Tajkistan");
        dtCountries.Rows.Add(5, "Turkmanistan");
        foreach (DataRow row in dtCountries.Rows)
        {
            countries.Add(string.Format("{0}-{1}", row["CountryId"], row["CountryName"]));
        }
        hfCountries.Value = string.Join(";", countries);
    }
}

I hope this will help you out.