To understand this endpoint you need to understand how we get a rate.
Basically it's how much of "secondary_currency" I need to get ONE of "primary_currency".
So, the instrument BTC/USD = 20000 mean that I need 20000 USD to get 1 BTC.
For this endpoint, if you want BTC/USD, the primary_currency_id is going to be the id of btc: 2.
And the secondary_currency_id is going to be the id of USD: 3.
To get the list of supported currency and their ids, check Get All Currencies endpoint.