How do I pass two parameter in the code behind to my Javascript Popup window URL

How do I pass two parameter in the code behind to my Javascript Popup window URL. Please help me fix my codes - I need to pass two parameters to my Javascript URL one parameter is the RowID and the other parameter is ADDNEW. I am able to pass one string but not the other, need help with formating my code behind.
Exampe: URL should be: Page.aspx?RowKey=112333&ADDNEW=M

CODE BEHIND   lkbtnAssigned.Attributes.Add("onclick", "return OpenWindAssignedVendor('" + LBLCode.Text + "');"); <--- HELP Need Two Paramter values

JavaScript
<script type="text/javascript">       var sF ...