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The REST API is not designed to return random records. You can get the specific record you request, you can get records in a specific sort order, or you can get records in default order.
If you want a random record, you will need a method of adding that randomness in your code. For example, you could do an initial query of only the record IDs for all the records in the table, pick a random record ID, and then request that specific record. If your table has many records, this may be very cumbersome.