Using web-console 

Go to My Servers > Manage

Scroll down.

Now, you can communicate with the web console by typing your commands: 

Once you type your command, press the Send button to execute the desired action.

Common commands

Discover all possible console commands by clicking Hint: 

Activating creative-mode

EXAMPLE

If you want to run your creative world and change parameters, you use command 

creativemenu. 

Type creativemenu > press the Send button

The outcome will be visible in the logs above the command line: