PhD in Sciences Po?

Hi everyone! I am thinking of applying to a PhD in Sciences Po but I am bit lost. I have heard that it is better to get in touch with a professor who agrees to your PhD project, then apply. Is that true?

Especially that I come from a different field: I did 2 yeard of prepa (CPGE), a business school (Grande Ecole) and worked for 7 years before starting this project. I am ok with applying to the Master's degree if I can do 1 year to help reach the PhD.

What do you think would be the best option for me? Thanks a lot!