How bypass this ?

Every time i am about to post a thread i get an answer while typing the message, maybe i explain it to myself better when i write it , OK so if you have seen this post it means i have not find an answer :)

I have a Silverlight app that passes a query string to an asp.net page that has an iframe that is hosting a third-party gateway page as depicted below

Code:

---

<form id="Form1" runat="server"  method="post" >
        <input type="hidden" name="p1" value='<%=(MerchantID) %>' />
        <input type="hidden" name="p2" value='<%=(GetNextTransID) %>' />
        <input type="hidden" name="p3" value='Purchase credits' />
        <input type="hidden" name="p4" value='<%=(Amount) %>' />
        <input type="hidden" name="m_1" value='<%=(Userid) %>' />
        <input type="hidden" name="m_2" value='<%=(Schoolid) %>' />
    <asp:Panel ID="Panel1"  runat="server">
        <table class="style1">
            <tr>
                <td class="style2">
                    &nbsp;</td>
                <td style="text-align: center">
                    &nbsp;</td>
            </tr>
        </table>
    </asp:Panel>
 
    <div id="layer1"
        style="position: absolute; width: 759px; height: 357px; z-index: 1; left: 88px; top: 233px">
 
       
        <iframe id="iframecredit" name="I1"    src="https://www.vcs.co.za/vvonline/ccform.asp"
            style="width: 744px; height: 333px; margin-top: 16px; margin-bottom: 0px;">

                Your browser does not support inline frames or is currently configured not to display inline frames.
                </iframe></div>
        <p>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <img alt="Ecash" height="609" src="images/BG.jpg" width="924" />
                </p>

    </form>

---

This payment gateway page accepts the parameters as you can see them there, so i have Properties that has values "After the Page load" so i cant get the values of the Query string before the page load. now the iframe will load with empty values, now i want to initialize the values before the iframe gets displayed

so my Page load will look like this

Code:

---

protected void Page_Load(object sender, EventArgs e)
        { 

                    m_SchoolId = Request.QueryString["SchoolID"];

                    //MerchantID
                    m_MerchantId =  Request.QueryString["MerchantID"];
                    MerchantID =m_MerchantId;

                    //GetNextTransID
                    Guid g;
                    g = Guid.NewGuid();
                    GetNextTransID = g.ToString();

                    //Userid
                    m_Userid = Request.QueryString["UserID"];
                    Userid = m_Userid;

                    //Schoolid
                    Schoolid = m_SchoolId;

                    //Amount
                    m_Amount = Request.QueryString["Amount"];
                    Amount = m_Amount; 
               
           
        }

---

I can step through this and i can get the values that i expect , but the problem is that the iframe URL get rendered first and obviously the values of the page load wouldn't have have values by that that time, how do i bypass this. any idea?

Thanks