The classic “poor man’s copyright” is to mail the script to yourself in a sealed envelope and put it somewhere safe unopened. Then if anyone steals your idea you have a dated postmark on the envelope to prove when you created it. You definitely don’t have to worry about the studio stealing your idea though. They have systems in place that keep the people who read spec scripts isolated from other creative employees at the studio for that exact reason. Lawsuits are way more expensive that low balling an option offer.

Other people have pointed this out but the major studios don’t really buy spec scripts anymore. They want something that comes with an audience already so you either need to build that somehow on social media or write for existing IP (but you’d need to license it yourself). Before a good apec script might have been adapted into an existing IP like your spy being rewritten as Jason Bourne, but since VC money bought out the studios they want more than a good script to produce a film these says.