Detroit, Michigan city tax returns are included in the TurboTax state program for the state of Michigan.
Were you a part time resident of Detroit in 2018? If so, you should have completed the city return when you were using the TurboTax 2018 state program for Michigan.
Form 5120 is the City of Detroit Part-Year Resident Income Tax Return (2018) - https://www.michigan.gov/documents/taxes/5120_642095_7.pdf
Form 5120 Instructions (2018) - https://www.michigan.gov/documents/taxes/5120_instructions_642111_7.pdf
