We designed the "Carrier Notes" field with this in mind. Anything in this field will show up on the Rate Con sent to the Carrier. This field will be labeled Carrier Notes or Driver Notes, depending on which version of Load Manager you're running. Just type your desired message and Save.
On newer versions of Load Manager, users have the ability to give instructions that are specific to each stop. Anything in the "Instructions" field will appear under the respective Pick or Drop on the Rate Con.