You must use the Symbols option if you want to insert symbols or special characters that are not available on the keyboard.
Go to Insert » Symbols » Symbol to view available symbols. You can see many symbols available there like Pi, alpha, beta, etc.
To use the symbol, select the symbol you want to add and click insert.
Using Special Characters
To see the available special characters, go to Insert » Symbols » Special Characters. There are many special characters available, such as Copyright, Registered, and so on.
To use the special character, select the special character you want to add and click insert.