AX 2009 Lookup methood help

Hi All,

I have created a table with 2 fields SalesId(EDT is SalesIdBase) and invoiceId(EDT is CustInvoiceId), and i have created a form and added these 2 fields, i am getting drop down for these 2 fields.

But i have a requirement  such that when i select SalesId in 1st field then in 2nd field i.e InvoiceId only invoices related to these sales id should come in drop down.

Please let me know how can i write lookup on form or any other way?

Thanks in advance.