Hi sew,
Sorry about the delay.
If the PhD explicitly suggest contacting the supervisor it is usually better to contact them than not. Contacting the supervisor beforehand helps you not only find out more about the PhD role but lets the supervisor get to know you before the interview.
An informal enquire would politely state that you are interested in the PhD role, explain your experience/degree and show some general enthusiasm for the project.
Goodluck,
rewt