As @johnpowell said, you have a lot to learn.
First question you need to ask: Are you the supplier of the data, or are you the consumer of the data? That’s a fundamental first step.
Assuming you are the consumer of the data, you are acting as the client and querying the server of the financial data using their API. So you don’t get to dictate how the API is constructed or how it works – you have to read their documentation and then build your software to make the API call.
For example, suppose you want to pull data from the New York Times. You would go to their API page link, see what APIs they have, then get a token for login.
Then you would study their API to see what data you need to send in the initial message, and what data they will send you back.
Then you need to write the GUI that will be used to display that data in your app.
If there is even one word in what I have written that you do not understand, then you are not ready.