Very interesting question. First of all, not necessarily you need an User Interface (UF) for performing any kind of simulations. You can perform any simulation via scripting as long as you can read and write parameters on an Open Database Management System (ODMS) such as Excel, Access etc. Creating UF is just for making things a bit more user friendly.
There are two methods of scripting in Ansys Workbench. The First method is using JavaScript language which is the language that is used for developing Ansys Mechanical. There is no published API available for this method and basically you are on your own for finding the functions required for scripting.
The second method is using Python language which all the APIs are published and available for scripting. You can find all of them in ACT help.
I have published couple journal papers and a book chapter and explained the process for the first method, however, the process is going to be the same for the second method as well in addition you have access to all the APIs since they are already published.