I had to add a letter of intent for one of mine. It was basically a paragraph in length (I think they said they wanted it to be brief though) and included some stuff about why I liked the area and why I thought the program would be a good fit for me.
Part of me thinks half this stuff is just to make some people reluctant to apply when they see they have to write a personal statement, provide an LOR, get extra immunization requirements, etc. I doubt the content will really be scrutinized. It isn't your ERAS PS, just write some stuff about what made you want to choose the away there and maybe why you're interested in your field and that's all that's really necessary.
