how to interact with my internal web application through an outlook add in

I have an internal website where we used to track and update our attendance. Now i want to develop an outlook add in through which I can apply the leave from the add in itself instead of logging into the internal website. We can assume that the credentials are same for both outlook and the internal application.

I am new to the whole concept of Add ins and I created a sample application and I think we might need an api or something like that to interact with the internal application. I need some solid examples to proceed further. I want to know if its possible and if so how to change the manifest and javascript accordingly.