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What is the etiquette for asking a professor to waive the prerequisite for a course?

Should I mail the professor? Should I meet him in person? I am a MS student in CS who wishes to take a graduate level course in Statistics department. The course is Machine Learning and I did cover most of the prerequisite course's content in multiple undergraduate courses so I want him to waive the requirement. I have budget for only 10 courses and wish to complete this degree in 2 years. Taking these prerequisite course would mean that I cannot take other courses that interest me.

The other reason I wish to have the requirement waived is because the prerequisite course itself is a statistics department course which also has prerequisite statistic courses and my college only allows me to take 2 courses outside of our department.