There are different ways to become a marshal. It differs per track or country how te be one. Sometimes it’s the circuit itself who orgnanize the volunteers (like in Assen). But it also could be a external organisation or a national association where you can apply. I will list a couple of webiste where you can sign up as marshal.

• Austria: Sicherheitsstaffel
• France: ACO
• Hungary: Sportbírói
• The Netherlands: TT circuit Assen, Zandvoort
• United Kingdom: BMMC, Motorsports UK
• Australia: Motorsport Australia
• Germany: DMSB, ADAC
• Monaco: ACM
• United Arab Emirates: EMSO
• United States: SCCA

If you know more websites where (foreign) marshals can apply for events like Formula 1, WEC or MotoGP, let me know!