How do I add a 3rd party app using API key?

[Donatsu]

Hi! I need help. I am working with a shipping company called Inpost, and they generated an API key token for me, but I have no idea how to implement it to my store, meaning how to embed this app. I know there is a shopify integration app for Inpost but it costs 19$/month and is not made by Inpost, they told me the app they gave me the API key for is free, I just need to embed the app (but again no idea how to do it).

Is this even possible? They never worked with shopify developers before so they cannot explain to me how to do it... I'm getting really frustrated, because I'm already paying for the deal we made, and can't afford additional 19$!