how to clientside validation in dhtmlx form

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ASP.NET

Javascript

Visual-Studio

I have text boxes and drop downs in my form.and I have a button named"search". when i enter the values and click on the search button I have display the search results.Entering all values is not required,user has to enter at least one value otherwise we have to give validation message that Enter at least one value. how can i do this validation?
I gave like:
var fn = myForm.getItemValue("FirstName");
var ln = myForm.getItemValue("LastName");
var c = myForm.getItemValue("City");
var st = myForm.getItemValue("State");
var cnt = myForm.getItemValue("Country");
var zc = myForm.getItemValue("Zipcode");
var em = myForm.getItemValue("Email");
var ph = myForm.getItemValue("Phone");
if (fn == "" && ln == "" && c == "" && st == "" && zc == "" && em == "" && ph == "" ) {
alert("please enter atleast one value");
}
my alert is working fine.but i want to display validation message in the form that"please enter atleast one field" at the top of the form.can u tel me how to this