As i said yesterday, you cannot.
But apprently you did not read my answer completely so i will continue here with a crystal clear exemple.
Create two tables in your database.
Table_A
id value
0 A
1 B
Table_B
id value parent
0 C A
1 D A
2 E B
3 F B
Create a report query that goes like this : select id, value from table_a
Create a report query that goes like this : select id, value from table_B where parent = '%STRING%'
%STRING% must be declare in your variable. You can create one by clicking on "manage variables" in the report object designer.
Create two drop downs in your form.
First dropdown, in report/plug-in id : set the name of the first report object
First dropdown, in onChange : ca_fdFetchSelectData(ca_fd.formId, 'IDOfYourSecondDropdown');
Second dropdown, in report/plug-in id : set the name of the second report object
Second dropdown, in report/plug-in id : set the report/ plug-in variable to : $({'STRING': ca_fdGetSelectedOptionValues(ca_fd.formId, 'IDOfyourFirstDropdown')[0]})
Hope this helps,