Fetching value of a filed from another Form

Suppose If I wanna to fetch the value of No. field in Sales Order form from the Purchase Order form, how shall I make it?

This makes no sense, unless you an define the link between the sales order and the purchase order. Navision does not do this, so I can see one of two scenarios that you are considering. 1/ You have made custom mods, in which case you need to explain th emods you have made. 2/ You are thinking of Special order or Drop ship, which are line based, not header based, in which case you need to know the exact link that is needed. Basically further information is required.