Given a playlist URI, click a button to subscribe.
/* Instantiate the global sp object; include models */
var sp = getSpotifyApi(1);
var models = sp.require('sp://import/scripts/api/models');
$("#subscribe").click(function(playlist){
var playlist = models.Playlist.fromURI("spotify:user:spotify:playlist:3Yrvm5lBgnhzTYTXx2l55x");
playlist.subscribed = true;
playlist.observe(models.EVENT.CHANGE, function() {
console.log("Playlist is subscribed!");
});
});
<input type="button" id="subscribe" value="Subscribe" />>