There are multiple places to message a Creative about a job. First, you need to create the role in the Jobs section which can be found in the top navigation. On the Jobs page click the "Create New Job" button on the top right. It will take you to the new job page where you quickly give us a few details about the job. Enter this info once and it will be sent to every creative you contact about this role. No need to copy and paste the details into every message. Once the job is created you're ready to reach out to people about it using the "Send Message" button in Search, on their MiniProfile or on their Profile.
Here is a video simulating sending a Creative a message about a job.