@Jimmy. Thanks again for your reply. Your use of the phrase "query parameter" eventually led me to the answer. Basically, I created a custom list with a PreviousPage text field, loaded the list in SharePoint Designer and created a custom default "New Form".
I then added a new server variable parameter: <ParameterBinding Name="ReferringUrl" Location="ServerVariable(HTTP_REFERER)" DefaultValue=""/> to the custom form.
From there, I edited changed the SharePoint form field to an ASP text control to pass the server variable results to: <asp:TextBox runat="server" id="ff5{$Pos}" text="{$ReferringUrl}" __designer:bind="{ddwrt:DataBind('i',concat('ff5',$Pos),'Text','TextChanged','ID',ddwrt:EscapeDelims(string(@ID)),'@ReferringPage')}"/>
I checked it a few times and later changed the text field holding the referring url to hidden.