B.Z.
My brother told my nephew at this age that it was "milk juice" in the sippie cup. My nephew drank it happily! Having your child use the sippie cup may rely solely on your terminology and enthusiasm.
You may also consider skipping the sippie cup for milk and going straight to the regular cup. Most 18 month olds have the motor skills to handle it, so why not? It can become a special thing to do at the table, and your child may get a lot of confidence by accomplishing it. Prepare for some spills by filling the cup with only 1 or 2 ounces to start. Have fun!