Are you saying that you neither have any idea how to do it, nor you know how to write any part of the code, e.g. you don’t know how to read a field value? Or do you have a particular problem but you forgot to explain what it is?
The first step is learning how to execute logic when the customer account number changes. Doing it just when a value is selected from a lookup would be a mistake, because the user can type in or paste the value as well. You should do it on modification, regardless of the way. Because the control is bound to a data source field, it will the best to do it there. For example, you can create an extension class for CustAccount field of SalesTable data source and create a CoC extension of modified() method. (An alternative is using an event handler.) When you know how to execute the logic, we can move into what we need to do there.