The block to launch a beacon is located in the bluetooth package. Once you've add that package to your project, simply drag the bluetooth advertise url block to the on start block. We also turn on the center LED as a visual indicator that the micro:bit is on.
bluetooth.advertiseUrl("https://makecode.com", 7, false) led.plot(2, 2)
You can also read about the details of the Eddystone support implementation in the Lancaster DAL.
By default, the micro:bit is configured to require pairing. Go to the Project Settings and enable the Unsecure connection mode for the beacon.
Flash your micro:bit and it'll be happilly beaming URLs.