Fetching data in header and line table based on text box

Look at the General Journal form as a place to start. Where the Batch Name is you could set this to blank or some value which will never exist each time the form is opened. Then this would be where the user enters or even looks up the record they want to display.

We have a form that has 1 textbox where an PO# is entered (scanned into)

It searchs the sales orders and if it finds a match it bings up that order.
If it doesn't find a match it then searches the posted invoices.
If it finds a match it then brings up that invoice.
else it says order not found.

We call it our quick search form.
Sounds like you are looking for something like that. or you can study the code to see how it works and then implement your own.

InvCount->Integer
SalesInvoiceHeader->Record->Sales Invoice Header
SalesHeader->Record->SalesHeader
ScanOrderNo->Code20
SalesForm->Form->Sales Order
InvoiceForm->Form->Posted Sales Invoice
SourceExp of textbox = ScanOrderNo

OnAfterValidate() of the textbox
IF ScanOrderNo = ''
THEN BEGIN
 MESSAGE('You Cannot Search A Blank Number!');
END ELSE BEGIN
 SalesHeader.SETCURRENTKEY("Document Type","No.");
 SalesHeader.SETRANGE("External Document No.",ScanOrderNo);
IF SalesHeader.FIND('-') THEN REPEAT
  InvCount := InvCount +1;
 UNTIL SalesHeader.NEXT = 0;
IF SalesHeader.FIND('+')  THEN BEGIN
  MESSAGE('Number Of Open Order(s) Found= %1',InvCount);
  CLEAR(SalesForm);        
  SalesForm.LOOKUPMODE(TRUE);
  SalesForm.SETTABLEVIEW(SalesHeader);
  SalesForm.RUNMODAL;
  CLEAR(ScanOrderNo);
END ELSE BEGIN
 SalesInvoiceHeader.SETCURRENTKEY("Sell-to Customer No.","External Document No.");
 SalesInvoiceHeader.SETRANGE("External Document No.",ScanOrderNo);
IF SalesInvoiceHeader.FIND('-') THEN REPEAT
  InvCount := InvCount +1;
 UNTIL SalesInvoiceHeader.NEXT = 0;
IF SalesInvoiceHeader.FIND('+') THEN BEGIN
  MESSAGE('Number Of Posted Order(s) Found= %1',InvCount);
  CLEAR(InvoiceForm);
  InvoiceForm.LOOKUPMODE(TRUE);
  InvoiceForm.SETTABLEVIEW(SalesInvoiceHeader);
  InvoiceForm.RUNMODAL;
  CLEAR(ScanOrderNo);
END ELSE BEGIN
 MESSAGE('Order Not Found');
 CLEAR(ScanOrderNo);
 CurrForm.UPDATE;
END;
END;
END;
CLEAR(InvCount);

It's one form with 1 textbox - then if calls already existing forms to pull the order you are looking for.

now that's just an example - I don't know the exact requirement you are trying solve.

I happen to put the textbox on a seperate form but you can try it anywhere you like.
All the code is OnAfterValidate of the textbox so it's all inclosed in one thing.

We use it to search posted & non posted orders - you might not need this - so you can use snipits if you like - it was just an example.