You just need an antenna of some sort.

See here: https://www.manualslib.com/manual/1185208/Pioneer-Sx-1280.html?page=6#manual

So a 300ohm antenna will work great. Like this: https://www.amazon.com/dp/B07BGY21GZ/ref=cm_sw_r_em_apa_i_vMzUEbQV3ZKV7

Since you have basically no FM antenna except for what is built in - this should improve your FM reception tremendously.

A simple 'T' shaped dipole antenna will do fine. You can even make one yourself and tune the length to match the wavelength of the station you wish to tune in.