Multiple Query String in Hyperlink asp.net

I have the following code:

string strUrl = String.Format("<a href='https://admin.modernroboticsinc.com/RmaMaintenanceCust.aspx?r={0}&Reg={1}'>{0}{1}</a>", Session["RmaNum"], Reg);
txtEmailBody.Text = txtEmailBody.Text.Replace("[CustLink]", strUrl);

This works fine.  My hyperlink displays like RMA123.  And when clicked, it displays the web page defined in the NavigateUrl property.  But now I want to add a second querystring but I still want it to display like it does now.  That is, RMA123.  But I want the second querystring on the NavugateUrl.  How can I do this?