The action to perform when clicked. Valid values are: 
<ul>
<li><b>open_url</b> Opens value as a URL in the player's default web browser</li>
<li><b>run_command</b> Has command entered in chat as though the player executed it themselves. This can be used to run commands, but the command will fail if the player does not have the required permissions to use it</li>
<li><b>suggest_command</b> When clicked the command is placed in the chat bar but not executed. Players may edit the command before running it.</li>
<li><b>change_page</b> Can only be used in written books. It changes to page value if that page exists</li>
</ul>