This is a webinterface for fm_transmitter (https://github.com/markondej/fm_transmitter) that can play youtube URL's.
https://moriceh.github.io/pifm-youtube-webui/
To install (I use curl because wget get a corrupted file...) :
curl https://raw.githubusercontent.com/moriceh/pifm-youtube-webui/master/Install.sh > Install.sh && sudo sh Install.sh
And reboot you PI.
To play a music, go to your PI's URL choose to broadcast a webradio or a YouTube video and go !
If you want to change the frequency for broadcast, go to your PI's URL and enter the frequency you want in the text box at the bottom. Click submit and there you go ! (The default one is 87.5)
To stop the broadcast, click on the stop logo on the page.