How can I log keystrokes of volunteers who visit my site (for science)

I'm working with a scientist, gathering information from volunteers who will have consented to having their keystrokes collected on our website. No private information will be collected. But I have no idea how to implement this, as I'm only a front-end designer. I'm not even sure I have the proper vocabulary here, though I've tried to make it as clear as possible.
Any help or direction will be appreciated.