Use the Clock to set the time to go off. Keep it running in incrementing by using the Clock.Timer event handler to play an mp3 alarm sound (using Player)
Your alarm will stop working if you use only the native Blocks when your screen turns off. Two ways to prevent the Screen closing without using a Foreground or Background method (which requires an extension) :
Using either an extension or a block routine drains your device battery rapidly . These work around methods are not running the app in the Background, they force the app
screen to stay open .
Try some Blocks and if you have issues, post what you have done here and someone will provide specific advice.
