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Download the tooltip package, and you can easily figure it out from the demo4.html.
Suppose the field element is <input id="field2" type="text" name="LastName" value="Mouse">, then if your validation before submitting the form has found field2 is not valid, you can call the popUpFailedValidation() function:
function popUpFailedValidation(){
tooltip.pop("field2", "#tip2", {position: 1});
}