Custom DFF on Standard form

Hi

Gurus/Experts,

How can I add the Custom DFF on Standard form ?

For Example : Document Category Form in R12.1.3.

Navigation > System Administration resp > Application > Sequential Numbering > Categories

Form Application : Application Object Library

Form Name : FNDSNDCT.fmb

In this form, standard DFF is not available.

Also, Attribute columns are not available in table : FND_DOC_SEQUENCE_CATEGORIES  .

I have add the context value and attribute column for DFF in this table using API : ad_dd.register_column  and  define the attribute column at DFF level as well.

Now I want to display the same DFF on the standard form . How can I achieve ?