Postgrest over foreign data wrapper

we have a tricky problem with my colleague.

We have to database db1 and db2 linked by a foreign data wrapper connection.

1 table "contractline" in db1 and "contract" in db2.

We use postgrest in order to request db2 via CURL.

But we want to add a link between previous tables.

In db2 we have a foreign table ft_contractline

example:

    Column    |           Type           | Collation | Nullable | Default | FDW options | Storage  | Stats target | Description

--------------+--------------------------+-----------+----------+---------+-------------+----------+--------------+-------------

 id           | character varying(1024)  |           | not null |         |             | extended |              |

 ccode      | text                     |           |          |         |             | extended |              | 

 status       | text                     |           |          |         |             | extended |              | 

We want to add for example, the following constraint:

ALTER TABLE contract ADD CONSTRAINT contractline_ft_contract_fkey FOREIGN KEY (contractid) REFERENCES ft_contractline(ccode);

in order to use the following query (via CURL on db2):

SELECT c.name, c.id FROM contract c JOIN ft_contractline ft_c ON c.id = ft_c.ccode WHERE c.type = 'business'

but we saw, isn't possible to add a foreign key on 'contract' table of db2 to 'ft_contractline' foreign table ...

Do you know way to do it ?

Thank all and best regards.

Adrien