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;
}