"To be eligible for a Letter of Consent, you need to be one of the following:

    An LTVP/LTVP+ holder who is married to a Singaporean or PR.
    An unmarried child (under 21 years old) of a Singaporean or PR, who holds an LTVP.

"

https://www.mom.gov.sg/passes-and-permits/letter-of-consent/...