When one uses “have to” + verb, it means that a person is obligated or must do something. For example, “I have to go to London next week for my job,” or, “I have to go to London next week to attend a wedding.”
Americans do not actually use “am to” + verb. This is a British expression. I cannot say with certainty that it’s something used today or only used in the past. It sounds very old-fashioned!