As it is written above. I came across that shortcut a few times but unfortunately I can't recall it because I found it not very useful. Now I've changed my mind and I was searching through web but without any success. Could somebody help me, please?
Your keyboard has curly brackets on it already. If it's like most, they'll be above the square brackets
If you wanted to be able to turn on automatic insertion of curly braces, this question may be relevant: http://stackoverflow.com/questions/12925282/is-there-an-auto-close-curly-brackets-plugin-for-the-geany-ide
Basically, you go to Preferences, then Completions, and the options are under Auto-close quotes and brackets.