Way 1:
function DoPostBack()
{
__doPostBack("txt_sssn_dt", "TextChanged");
}
Calling ASP.NET server-side events using JavaScript
Way 2:
Calling Server Side function from Client Side Script
Way 3:
one of the way to set focus lost like this
function texboxchange() {
var txtBox = document.getElementById('<%= TextBox4.ClientID %>');
var count = txtBox.value.length;
if (count == 2)
{
document.getElementById('<%= TextBox12.ClientID %>').focus();
return true; // this will call textbox changed event.
}
}