See below..
<asp:button id="btnsendrequest" runat="server" text="Submit" onclientclick="return validaterequirment()" xmlns:asp="#unknown" />
Javascript code below..
function validaterequirment()
{
if (document.getElementById("<%=TextBox1.ClientID%>").value=="")
{
document.getElementById("<%=TextBox1.ClientID%>").Style.add("border","solid 1px red");
return false;
}
}
Write above code in your master page. It will work in all content pages. TextBox1 is id of textbox. If y=textbox1 value is empty its border color become red and avoid postback.