How to Pass form values to API ?

Hi have a simple form (login and password) and on button submit it should call an API to check if the user exists, the response i am getting is JSON. But i am not sure how to pass the textbox values to API and how to check in API response?

Can anybody please help with an example...

i tried calling API on button click, form submit, tried with backed query section but nothing working, seems i am doing all wrong

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