Is it possible to "Make yourself" dream of something in particular?

I've heard that if you focus on one thing really hard when you're falling asleep, if you have a dream that night, it'll most likely be on what you were focusing on. I've also heard that you can induce nightmares, if you really wanted to, by doing all these weird things (eating this potium vitamin or something, thinking of things that scare you, etc.), but I honestly wouldn't know if that works, lol.