How to add record in multi reference field table?

How does one add both these fields into a new record using POST? I'm struggling to find a way, as having the user_id and sale_id on the frontend isnt an option. Is it possible to use a universal id? As this is a dashboard for a user I was thinking the logic would somehow look like:

user Auth required

input: universal_ID

if input.universal_ID == sale.universal_ID

then: Add Record in user_dash

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