For within your app? You shouldn't do that because you'd need to use the web API to set the custom license field, and thus you'd need to include the API key in your app.
For your web servers? Use the http://wyday.com/limelm/help/api/limelm.pkey.setDetails/ web API function. Also see the web API pack on your API page for full C# examples.
If you explain exactly what you're trying to accomplish I'll be able to give you a better answer.