I\'ve just created a custom order form on the Microsoft Dynamics CRM Online.
Basically, I\'d like to update Total Amount field as multiply of Quantity and Amount Field.
Here is info on how to setup the javascript for your forms: microsoft's form programming reference
the code for your calculation will look something like this:
var amount = Xrm.Page.getAttribute("youramountfieldname").getValue();
var quantity = Xrm.Page.getAttribute("yourquantityfieldname").getValue();
var totalamount = amount * quantity;
Xrm.Page.getAttribute("yourtotalamountfieldname").setValue(totalamount);