How change Finance API to custom?

Hi there!
I am wondering if I can get any directions on how to change the Finance API to custom?
I need the exchange rate resource to be ECB (European Central Bank), is this possible?

It says
“If you select “Custom”, you can custom exchange rate via “wmc_get_currency_exchange_rates” hook”
but I’m quite lost on how to go about this!

Would appreciate any help, thank you very much! /Caroline