Firefly III [0] solves this by making the user responsible for this. They expect you give it the necessary details (api keys etc) for Spectre, which it then uses to download transactions. Could you not do something similar? Write a small guide for users on where to register and what to do, and give them config options to put in the necessary API Keys, and then use that to download the transactional data?

[0] https://docs.firefly-iii.org/firefly-iii/importing-data/intr...