Hi,
Please check the user interfaces in the below folder :
C:\Users\Public\Documents\National Instruments\TestStand 2019 (32-bit)\UserInterfaces
There are two types of user interfaces with source code shared.
Simple OI which has a simple UI and Full features for with advanced features.
You can run the .net user interface which was developed using .NET API.
You can modify the code to your requirement.
Ravi