Textbox focus problem in ASP.NET, C#

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C#

I have 4 Textboxes in a payment_transfer screen:

==============================
debit_customer  : TextBox1
payment_number  : Textbox2
credit_customer : Textbox3
amount          : Textbox4

In this, every time I entered the value in textboxes and click on screen, the page gets refreshed and the cursor will be focused to debit_customer textbox every time.

Now my PROBLEM is Textbox Focus Problem...
When I give the credit_customer value and click on screen and screen refreshes, then the cursor should be focused to amount textbox.

Please help me.

Thanks.

What I have tried:

protected void Page_Load(object sender, EventArgs e)
        {
            Master.InfoMsg = "";
            if (!Page.IsPostBack)
            {
                Master.Title = GetStrResource("pmt_trn");
                Master.FormId = 742;
                hdnuserid.Value = Master.UserId.ToString(); 
                DataBind();
                paymentfield.Visible = false;
                tdsfield.Visible = false;
                this.btnsave.Attributes.Add("onclick", 
                "javascript: if (validateForm()){this.disabled = true;}
                 else {return false;}");
                this.btnstage.Attributes.Add("onclick", 
                "javascript: if (validateForm()){this.disabled = true;}
                 else {return false;}");
                uxPayDt.Value = Convert.ToDateTime(Session["VALUEDATE"]).ToString
                (Master.GetProfileSetting("DATEFORMAT"), 
                 CultureInfo.CurrentUICulture);
                /* Drop down code*/
                AppvaluelistSvc vl1 = new AppvaluelistSvc(Master.userInfo);
                DataTable dt1 = vl1.GetValuelistValues("PMTTDSTRANSFER");
                if (vl1.status.errcode != 0)
                {
                    vl1 = null;
                    return;
                }

                if (dt1 != null && dt1.Columns.Count > 0)
                {
                    dt1.Rows.InsertAt(dt1.NewRow(), 0);
                    transferdropdown.DataSource = dt1.DefaultView;
                    transferdropdown.DataValueField = dt1.Columns[0].ColumnName;
                    transferdropdown.DataTextField = dt1.Columns[1].ColumnName;
                    transferdropdown.DataBind();
                }
                SetGridsSource();
                if (Request.QueryString["PYMT_TRANSFER_ID"] != null 
                    && Request.QueryString["PYMT_TRANSFER_ID"] != "0")
                {
                    Master.Title = GetStrResource("edit_pmt_trns");
                    hdnpymttrnsid.Value = 
                                  Request.QueryString["PYMT_TRANSFER_ID"];
                    if (Request.QueryString["REF_TXN_TYPE"] != null 
                        && Request.QueryString["REF_TXN_TYPE"] != "0")
                    {
                        transferdropdown.SelectedValue = 
                                         Request.QueryString["REF_TXN_TYPE"];
                    }
                    SetValues(Convert.ToInt64(hdnpymttrnsid.Value), 
                              Request.QueryString["action"]);
                }
                else
                {
                    Master.FormMode = "I";
                    BindDetails(1);
                }
            }
            if (custnbr.Value != "" && custnbr.Value != null )
                SetCustomerName(custnbr.Value);
        }