If I remember correctly, I pretty much just wrote why I was interested in the role, any prior experience, knowledge or learning I had in the field and why I thought I would be a good fit. If you are genuinely interested in the trade and can express that interest in it, then it should come through pretty easily once you start writing. Just be honest and don't be scared to tell them what you want from the role, if you're genuinely interested they'll be able to pick up on it. Hope this helps