Need help writing an Bank API.

Please tell me how I can start and where to start with the bank installment plan, it seems like it’s not difficult for the first time, I can’t understand.

This does not look like a convenient endpoint as it uses client dynamics rather than server to server. First see if they have a RESTful API.

Here is a classic example for an API request:
https://wiki.litecart.net/how_to_create_a_payment_module#using_classic_api_requests