Default.aspx.cs

ContentPlaceHolder cp = (ContentPlaceHolder)Page.Master.FindControl("CPH");
TextBox tbName = (TextBox)cp.FindControl("txtName");
TextBox tbAge = (TextBox)cp.FindControl("txtAge");
string script=string.Format(@"function IDIssue() {{ var txtName=document.getElementById('{0}'); var txtAge=document.getElementById('{1}'); alert('your name is '+ txtName.value+' and age is '+txtAge.value); }}",tbName.ClientID, tbAge.ClientID);
ClientScript.RegisterClientScriptBlock(this.GetType(), "IDIssue", script, true);