In this case, the whole logic changes. But to make a shortcut, you can follow these steps:

Create a new field (formkey) in the DEPT table.

Create a new field (igkey) in the EMP table.

Create a sequence deptfrmkey.

On the dialog screen, for the DEPT table, make this field hidden and set the default value as sequence and give the name of the sequence. Also, you can use the SQL query, for example, select deptfrmkey.nextval from dual.

In the EMP grid, synchronize columns, and make the igkey field hidden and set the default value as item DEPT table formkey.

Change the before submit DA as following

var n_deptno;

n_deptno = $v("P19_FORMKEY");
var model = apex.region("igemp").widget().interactiveGrid("getViews", "grid").model;
model.forEach(function(igrow) {
     model.setValue(igrow, 'IGKEY', n_deptno);
});

Now for new records, the default value will populated for DEPT table and it will be copied to EMP table igkey.

Now create a before insert trigger on the EMP table to get the deptno on behalf of the igkey. Below is an example:

Create or replace trigger trg_bi_emp before insert on EMP for each row
Declare
   n_deptno dept.deptno%type;
Begin
   select deptno into n_deptno from dept where formkey = :new.igkey;
   :new.deptno := n_deptno;
End;

This is the tested method and works perfectly.

Please let me know for any further questions.