i have two items in the ComboBox so i want the user to be compulsary to choose item before he/she click Submit.

OnClientClick="javascript:return ValidateSaveRegion()

this is javascript call above validateSaveRegion()in your submit button and write this below code in aspx it wil work...try

function ValidateSaveRegion() {
  var Name = document.getElementById(cntPlcHolder_CmboxName;);
          
           if (Name.value == "";) {
               alert(Enter Name;);
               Name.focus();
               return false;
           }

if (ComboBox1.SelectedIndex!=-1)
{
//Go Further
}
else
{
MessageBox.Show("Please select");
}