API Program build

Hello, we are needing a basic API connection built to connect to an external third party database. We work with patient data in a health care setting. Our goal is to be able to have a program that will be able to allow us to import an excel spreadsheet with patient data such as name, date of birth, and medical ID number and then connect to the third party system and get back "out of pocket cost" per patient. I believe the "out of pocket cost is a single column on the third party system. We desire the program to be written in C# if possible. We are working with this third party team and if you choose to help us, you will be expected to be on at least 1-2 phone calls with the third party company we are working with to gather whatever information you need to make the API program work properly. Once the program is complete, we will need the source code of course. Thank you