Random number generator code

So I am trying to make it so that there is a 1/1000 chance of a random event happening. I thought this would work, but it doesn't, and I am unsure what to do next. I used google, but only found code that worked in tens 10%, 20%, etc. I don't know where I can find a way to make the chance smaller than 1/10.

Current code I am trying to use: https://imgur.com/a/rPoS9pI

If anyone can help, it would be appreciated.