Add a buy button to your tracks and playlists by clicking the pencil icon below the waveform. In the edit page, go to the 'Metadata' tab and add your Juno artist, label or release page for listeners to purchase your release / releases.

If you are a Pro user, you can also customise the text used for the 'Buy button' on the edit page in the 'Buy link title' section, to read 'Buy at Juno'.