This is a third-party script/app and is not created by the WaniKani team. By using this, you understand that it can stop working at any time or be discontinued indefinitely.
As requested in the thread for the mention button [Userscript] Forums: Easy Mentions:
What
This script adds a button to the bottom of each post. Clicking this button is the same as clicking the button in the reply box. Essentially all it does is click the reply button on the post, wait 100ms, then click the quote whole post button.