It should work out of the box, but it hasn't been tested for a long time (only briefly when the feature was added).
First of all make sure you are copying plain text data only by copying some text from notepad. Although I doubt it is the case, I think it is technically possible for a program to only put the formatted text in the clipboard without including the plain text.
What OS and compiler are you using?
Edit: could you also show a minimal example code that I can just copy-paste to test with?