Show fields on second form on the basis of SalesId selected from First form

Hi everyone, 

I have created a form with data source CustInvoiceJour table. It is showing some fields from the datasource on the grid like SalesOrder, InvoiceAccount, InvoiceAmount, CreatedDate and DueDate. On top of the form, I have a dropdown for SalesId. Under it, I have a button. 

Then, I have another form with datasource CustInvoiceTrans. There is a grid on this form as well. The grid has different fields like ItemId, Name, Qty, Price, Unit, SalesId, InvoiceId. 

The problem is, we have to select a SalesId from drop-down from first form and press button. The second form will show the records on its grid on the basis of the Id selected in the first form. 

I have read about Args class but all of the existing solutions are using the same datasources on both forms. My forms are using the different datasources. I select SalesId from dropdown and when I press button, The second form opens but without any data.

Any help....Thanks ALL!