Hi,
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 so you cannot add your own, if you put a form inside an html content feature for example you are really putting a form inside of another form that is already there and nested form elements is not valid html.
The solution is to make a simple custom module using an <asp:Button , you can set the PostBackUrl property on the button and that will set the action on the existing form when the button is clicked.
See the article Building A Simple PayPal Buy Now Button which shows how to do this since the PayPal button needs to post to an external url.
Hope that helps,
Joe