Michael,
Since it's a server control, why don't you set it in the Page_Load event in the codebehind:
string param = Server.UrlEncode("1010011123-00");
HyperLink2.Attributes.Add("onclick", String.Format("return dnnModal.show('{0}?parameter={1}&popup=true',true,{2},{3},false);", "/askaquestion.aspx", param, 350, 800));
That should give you the control you need to inject the value from a module setting, user input, or querystring.