Cannot hit REST API from a web app.

Hi all,

I am developing a web app to have dashboard which displays all the tickets of current logged in user and his/her group. 
My thought process is to get a session key with username and Application API key, and use that session key to get list=mine and list=group in my rest call from app.

The issue is this works in postman but not via the app. I get Authentication failure in the response body. Is there something that I am missing? If someone has implemented something similar in past, I would appreciate if you can guide me. Any help is appreciated.