how to capture Submit button on iProcurment page

Hi,

I have a requirement to extend standard OAF page of iProcurement, if user click submit button it should do some validation first instead send it for approval and showing requisition number.

I have disable this button successfully and that is happening in my custom controller code but when i try to keep it enable and try do some validation it is not happening it is suddenly directing me to a message your requisition 1101111, Continue shopping.

Please suggest how can i do some validation on this button click event.

Thanks.