i need to enable requirfield validator via javascript

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Airbooking.aspx.cs" Inherits="Airbooking" %>

<%-- --%>

REQUISITION FOR AIR TICKET BOOKING

From Place

To Place

Date Of Journey

Flight No:

Departure Time

ErrorMessage="*">

ErrorMessage="*" Enabled="false">

ErrorMessage="*">

	Prno	Name	Designation	Mobile No
1
2
3
4
5
6.

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texbox(txtfrom1,txtto1,txtjd1,txtflno,txtdep1) requirfield validator always enabled.(txtfrom2,txtto2,txtjd2,txtflno2,txtdep2 are requirfield validator disabled ) then customer entered second flight detail txtbox2 a single letter (txtfrom2,txtto2,txtjd2,txtflno2,txtdep2) requirfield validator automaticaly ennabled in javascript help me