In your deck, go to Browse, then press enter if it doesn't display all cards already, pick a random one, press the "Card" button.
In the "Front side" part, replace the original text :

<a href="http://kanji.koohii.com/study/?framenum={{text:Heisig number}}" rel="nofollow">{{Keyword}}</a>

with the following :

<a href="http://kanji.koohii.com/study/kanji/{{text:Heisig number}}" rel="nofollow">{{Keyword}}</a>