function Check()
{
if (Page_ClientValidate())
{
if (document.getElementById("ctl00_gbtsContent_chkAgree").checked == false)
{
alert('Please select the checkbox.');
return false;
}
}




}


CssClass="MenuButtonlink" ValidationGroup="Save" OnClick="btnSave_Click"
OnClientClick="return Check();" />

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http://alvinzc.blogspot.com/2006/10/aspnet-requiredfieldvalidator.html