You are asking how can you direct the CSE to execute compiler instructions like [assembly: AllowPartiallyTrustedCallers].
This is an assembly attributes and is not a separately-executable C# statement.
This assembly attribute is actually a compiler directive that will only be acted upon when the C# source code for an assembly is compiled by the Microsoft JIT compiler.
The CSE is designed to provide the APL+Win programmer the ability to execute C# executable statements.
The CSE is effectively a line-by-line C# statement interpreter based on the Microsoft Roslyn technology.
When the CSE 'Init' method is used the C# side of the CSE creates an in-memory assembly.
The APL+Win/CSE programmer cannot modify the attributes of that in-memory assembly.
When assembly or module attributes are needed, I suggest that you use Visual Studio to create your assembly, build it, giving it a 'strong' name if you are using the AllowPartiallyTrustedCallers assembly attribute, and then use the CSE 'LoadAssembly' method to load the assembly into the context of a CSE instance.