Can I Use Slang in the Speaking Section for IELTS?

We were inspired by an amazing question. Can I use slang in the speaking section for IELTS? Apart from that, what can I use in my speaking sections such as idioms, phrasal verbs, etc. The first phrase you will learn is informal. And then next, we'll get to the formal one. So here's the informal one: "I am beside myself with excitement"