How to write code in 'Enter a C# expression' in PWA Visual Studio workflow

So I created a variable by right-clicking a property 'EnterProjectStage'. Now in the variable created, there is a property called 'Default' and it says  'Enter a C# expression'. I believe this property is to write a c# code, if so, I need to write a code to bring a value from an xml file which is created inside the solution itself. How this code can be written?

Please help.