When a radio presale is announced, a radio station will usually tell listeners that they will broadcast a special code during a certain time frame, or they will make it available at their website. Listeners then take that special code to Ticketmaster's website and use it to by tickets before they officially go on sale to everyone.
Radio stations like radio presales because it encourages people to listen to their radio station. They usually will not give the exact time of the code being announced, instead they'll tell listeners to listen all day or over a certain time.
Going against their intent, many websites are dedicated to putting up radio station presale codes the second they are announced on the air or put up on the radio station's website. People use these sites to get radio station presale codes without having to listen to the radio station.