The Tab key is designed to move from one paragraph to another and there's no way to make it work like a standard word processing Tab key.
You can, however, create a macro with blank spaces to use a keystroke to create an indent:
- Go to Document > Macros;
- Click NEW. This will create a new Macro that you can title anything you want; in this case let's name it "indent." (Note: Mac users will need to modify an existing Macro as there isn't a way to create a new one);
- Assign a Keyboard shortcut -- one that is not used by another function, like Control+Alt+T;
- In the Macro Text box, press the space bar 5 times or however many spaces you want the indentation to be;
- Click OK and on your document, press CTRL-SHIFT-T. You will see the indentation.