Many thanks to SWLing Post contributor, Adid, who writes:

Check this little gadget for android phones or tablets

https://www.aliexpress.com/item/32976847250.html

If they have the FM app installed than this will come handy.

One needs to select speaker and not earphone as expected.

It works fine here. (I don’t know if Apple phones have an FM module)

Thank you, Adid! Very cool! This little antenna costs $0.90 USD–an insanely low price.

If you have an old set of earphones lying around the house (I probably have two dozen that ship with various radios) you could also snip off the earpieces and use the pigtail as a wire FM antenna.