create a link button in this
<asp:linkbutton id="LinkButton1" runat="server" postbackurl="<%#"~/Guest/Default.aspx?id="+Eval("patientID") %>">Add To Cart</asp:linkbutton>
then on next page you can retrieve this value on label id on page load like this
string id = Request.QueryString["id"];
now pass this id in your query and then retrieve your value in text box and dropdown