Should I contact a supervisor before applying?

Short answer: It helps to know whether a group has space for your topic, but selection is by the committee, and a professor cannot promise you a position.

• Subject line: "Prospective PhD applicant, [topic], [call name]".
• Body, five to eight lines: who you are, your degree, the one idea you want to work on, why their group, CV attached.
• One polite follow up after two weeks is fine. No reply does not mean you cannot apply.