Google Chromecast audio normally is not able to accept directly radio streaming address but there is a nice and easy way to send radio streaming links to Chromecast audio using an android phone/tablet with the RS_player APP that you can download from here.
RS_player is easy to use. You need to have an account in www.iptv-updated.com and after you can organize your country groups adding manually the radios with the links OR search the radios from the internal search engine.
You have to organizes the radios in R_player only once. After you can access to the same configuration from all yours Android mobile devices.
All channels added in your account (created from you or added from internal search engine) are also visible in the Android RS_player app (in the tab remote channel, after that you are logged in). If you “take” this channel in the local playlist, you can play the channel to chromecast device.
You can find here the detailed user guide of how to add and search the channel in RS_player.