Hi,
The problem is that in ASP.NET WebForms which mojoPortal is built on, there can be one and only one form element in the page and one already exists wrapping the whole page so you can't add one inside it. However the problem can easily be solved by developing a custom usercontrol and using an <asp:Button you can set the postbackUrl property on the button to the remote url.
There is more detail on this in the article about Building a Simple PayPal Buy Now Feature.
However for your scenario where you want to open a popup page I'm not really sure of a good solution, possibly you could load a separate html page with the form inside an iframe.
Hope that helps,
Joe