C# - Modifying The Query String On Dropdown Index Change In Code Behind?

I have a drop down box that is in the edit template of a formview. I want to be able to add a parrameter to the querystring when the drop down selected index is changed.

I tried

Request.QueryString.Add("tabindex", (sender as WebControl).TabIndex.ToString());

But I got an exception saying the collection is readonly.

Here is my markup

<icms_ref:ReferenceDropDownList ReferenceDataManagerProviderName="ROCSQLReferenceDataProvider"
ID="ddlEnquirerHearType" TabIndex="2" runat="server" ReferenceSetName="EnquiryHearType"
AutoPostBack="true" OnSelectedIndexChanged="EnquirerHearType_SelectedIndexChanged"
DataTextField="ShortName" DataValueField="ReferenceId" />

and here is my code behind.

protected void EnquirerHearType_SelectedIndexChanged(object sender, EventArgs e)
{
var pnlEnquiryHearTypeOther = fvEnquiryInformation.FindControl("pnlEnquiryHearTypeOther") as Panel;
pnlEnquiryHearTypeOther.Visible = DdlEnquirerHearType.SelectedValue == ((int)EnquiryHearType.Other).ToString();
ResetTextBox("txtEnquiryHearTypeOther", fvEnquiryInformation);
Request.QueryString.Add("tabindex", (sender as WebControl).TabIndex.ToString());
}