As far as I know, there is no mba preparatory course in Stanford. You can check it in official website. What if Stanford doesn't? Change schools, of course, but look at this question. To be honest, I don't think you can apply for a prestigious school. Let's be realistic. Applying for a prestigious school is more difficult than you think. Don't waste your efforts.
Not every university has an mba preparatory course. It seems that only ordinary universities have this kind of preparatory course, and each university has different requirements. Take La Verne University as an example.
The entrance requirements are TOEFL 70, GPA3.0, and the course can take 18 MBA credits. After the language meets the requirements of the school, it will be transferred to the formal MBA course.
As for how to apply online, of course, you can check in official website, and all the information you need is under Apply.