You are not sure if an employer can ask your salary history. The job application asks you to enter your salary history. Is it legal in the USA to ask a candidate about their previous salary history?
Maybe. For some states, the law only applies to the state government. It depends on the state, the city or possibly the type of employer. Read this article (but please note that it has not been updated in the past six months, and this is not legal advice). You may also want to see Paycor.com and Ultimatestaffing.com for their articles on this subject.