Energy credits are a large part of the Stellaris economy. They're very easy to add in via cheat commands. The command you must use to cheat energy credits into the game is the cash command (more info).
You'll first need to open the console, you can do that by pressing ` or ~. If you need more help opening the console, click here.
Simply typing cash into the console and pressing ENTER will add 5000 energy credits to your balance. Adding a number as an argument to the command will add that amount of energy credits, for example: