I have salary income and let out a jointly owned HK property, where should I report the rental income?

Part 3 of the Individual tax return (BIR 60) caters for SOLELY-OWNED properties only.  You should use a Property tax return (BIR 57) to return jointly-owned property income.  In the case of company-owned property income, the form is BIR 58.

Reference: https://www.ird.gov.hk/eng/faq/pa.htm