Freedom UI: Dropdown populated from an arbitrary object with a page-parameter filter

Hello Creatio Community,

How can I create a dropdown on a Freedom UI form whose options come from an arbitrary object (for example, Contact) and apply a custom filter that uses a page parameter? 

For example, populate the dropdown from Contact, but only show records that match a condition involving a page attribute (via Relationship table where AccountAId is AccountId on the page and ContactBIs is field to populate in the dropdown). What’s the recommended approach? A small code example would be great.