How can i create a Index on an Array?

I have a query which looks for email address in the send_to key of my doc which looks like this.

SELECT META().id, c.*
FROM Contacts c
WHERE c._type = 'track_request'
    AND ARRAY_CONTAINS(c.send_to, '')

its a bit slow with close to 1 sec query time and the Advise recomends the folowing Index

CREATE INDEX adv_array_contains_send_to_eoko2dko3dko3google_com_type ON Contacts(array_contains((send_to), '')) WHERE _type = 'track_request'

But i dont want to have an index specific for that address but for any address

CREATE INDEX ix1 ON Contacts (DISTINCT send_to) WHERE  _type = 'track_request';

SELECT META(c).id, c.*
FROM Contacts c
WHERE c._type = 'track_request'
    AND  ANY e IN c.send_to SATISFIES e =  '' END;

You can use Range, LIKE also with above syntax. If required leading wild card check Example 4