curl "https://devxxxx.service-now.com/api/now/v1/email" \
--request POST \
--header "Accept:application/json" \
--header "Content-Type:application/json" \
--data "{
              \"to\": [
                  \"gareth <gjones@ca.ibm.com>\"
              ],
              \"subject\": \"Hello There\",
              \"text\": \"Test Message\",
              \"html\": \"<b>Test Message</b>\",
              \"headers\": {
                  \"X-Custom\": \"header\"
              }
          }" \
--user 'admin':'admin'

with xxx replaced and password added.

thanks!