How to notify Onsuccess in Ajax.BeginForm as false.

public PartialViewResult AddBranchContact(BranchViewModel objBranchContact)
{
    List<BranchContact> BranchContactList = new List<BranchContact>();
    if (ModelState.IsValid)
    {
       if (Session["BranchContactList"] == null)
       {
           objBranchContact.BranchContactDetails.BranchContactID = 1;
           BranchContactList.Add(objBranchContact.BranchContactDetails);
           Session["BranchContactList"] = BranchContactList;
    return PartialView("PartialViewAddBranchContact", BranchContactList);
       }
       else
       {
    List<BranchContact> ContactList = Session["BranchContactList"] as List<BranchContact>;
           ContactList.Add(objBranchContact.BranchContactDetails);
           Session["BranchContactList"] = ContactList;
    return PartialView("PartialViewAddBranchContact", ContactList);
       }
    }
    else
    {
       return PartialView("PartialViewAddBranchContact");
    }

@using (Ajax.BeginForm("AddBranchContact", new AjaxOptions { HttpMethod = "POST", OnSuccess = "$('#myModal1').modal('hide');", UpdateTargetId = "myTable" }))
{}