Callback in Rest Api's

Question

Hi,
My question - from a 3rd party, my rest api will be called and for that we need to send a response after process (which might not be sent instantly), the response might take a day or two even. (Maybe a webhook or reverse api concept not sure). Need to make sure for that specific call we are returning the response(maybe use some kind of unique identifier).
Cannot use async/await as all threads can occupied if there are a lot of calls and it cannot send a response after few days.
Found some information on this link - https://metamug.com/article/callback-api.html
Need some help for implementation.
Please let me know if something else required.