graduate admissions - Applying to a PhD in Machine Learning without much knowledge of the subject

Lets suppose that I see an open PhD position in the field of Machine Learning, area whose I have only some very basic or general knowledge, but that I am willing to learn more about it. I wonder what I should put in my cover letter to show that I have a genuine interest in the subject and that I am willing to learn more about it. I mean for what I know, the first year in a PhD program is for a student to gain knowledge about the theoretical parts that will help him or her to continue their PhD studies. I have seen that also some of them receive aome training that will help them to narrow their interests into one specific topic before the writing of the thesis start.

I am somewhat traumatized after once I applied to a PhD position and in the interview when I mentioned that "I was willing to learn more, enroll into courses and being self taught for the research that they were making"; and the professor in charge reply to me in a harsh way "You are not here to begin to learn, we need people that already know what we are doing". That shocked me somehow.

So bottomline, how can I express in a cover letter my genuine interest in learning the field in which the research group is working, without being rejected?

Thanks