Follow the prompt to write your OptomCAS essay. If you want to, (but you don't have to) you can customize it to each school. For example, you might want to say, "I've always wanted to go to SUNY because the Dr. I shadowed went there and told me how great it was..." or "I want to attend Salus because it's very important to me to stay close to my home in Philadelphia due to...." If you customize them, name them after each school (example: "SUNY essay", or "Salus essay") so OptomCAS sends them to the corresponding schools. If you don't customize, name it after yourself and upload this essay to all of your chosen schools. You aren't giving it a title, like "Optometry in the Twenty-First Century; Why it's Right For Me" (I know, how lame), You are giving it a name. Plus you'll notice they only give you about 20 or so characters to name it.
If the schools want another, different essay, they'll let you know in the supplemental application.