[WindowsCentral.com]

Best answer: Microsoft's Surface Pen works well with the HP Envy x2.
Microsoft: Surface Pen ($100)
It just works

The Surface Pen is an excellent accessory for any Windows 10 tablet that supports it. Lucky for HP Envy x2 owners, Microsoft's handy drawing tool works just fine with this device too. The pen itself uses N-trig, which is fine for the Envy x2, allowing you to not only pair the two together and use them without issue, but the button also works.

Full story from the WindowsCentral blog...