How do we phrase a job ad so we don’t discriminate against foreign-born workers?

Q: “I have a client with 40 employees that is not able to offer sponsorship to any candidate who is not legally able to work in the United States. I understand that we cannot put 'Prefer US citizen or authorized to work in the U.S.’ on a job ad, but is there any means that we can use to alert candidates to the requirements?” – Anne, District of Columbia