Setting up remote web application without AD

I have bought a windows server 2019 and would like to give some users access to a accounting system, and only that application. We are talking of max 5 people, so I do not want to setup an AD for this matter. After installing the terminal server and its needs, I am able to let the users into the server and they can run the application. I am not able to setup a remote App without AD. Is there a way of doing that? I have tried running the mstsc command but then the application is not responding as it should... some buttons are missing...